MP=Motoplane (in Jr.Scout frame)
VT=Vertical Twins excl Arrow.

Answers:
Motor is 1946 Chief, Serial# 6403 (6303rd* made!)
Frame is 1941 Four, Serial# 1089, (989th* made!)   *as first machine started at xxx101

In order for a cycle to have ‘matching numbers’, the frame number must match the engine number and the first three numeric digits of the frame number equal the decoded motor number. i.e. for our example, the frame would have to read 346 6403.  see http://www.indianmotorbikes.com/resources/serials.htm#frnu m">Frame Numbers section  http://www.indianmotorbikes.com/resources/serials.htm#to p">Top

Examples:
AH1088 is a 1926 Big Chief, 74ci, serial 1088
(similarly, AZ1088 would be 1926 61ci)
BGP1055 is a 1927 Scout, 45ci, serial 1055
(similarly, BG1055 would be 1927 Scout 37ci)
EA779 is a 1930 Four,  serial 779
(similarly, BL779 would be 1926 Prince) http://www.indianmotorbikes.com/resources/serials.htm#to p">Top

(I, L, N-Q, U & W  not used.)
also see http://www.indianmotorbikes.com/resources/serials.htm#n1 2">note 12 for ?Pxxx models (where ?=R,S,T or V)

Example 1 – 51X107
Take the engine number 51X107 which breaks down to:
5 is Scout 37″, (because of X) then because the second number is not zero (as it would be if serial < 1000)  we add it (1000) to the serial 107 making it 1,107,
The X is the year 1924. So it is a 1924 Scout with a engine serial of 1107…
eg.  a serial of 54X155 would be the same model and yr with a serial of  4,155.
similarly, a serial of 67X157 would be the same model and yr with a serial of 17,157.Example 2 – 72M911  
Take the engine number 72M911 which breaks down to:
‘72 equals 7 &2 and  think of this as 2000 (thousandths digit)
The M is the year 1919. The 7 is a 61ci Powerplus (because of  the M) and the remaining 2000 is the thousandths unit of the engine number whereby adding it to 911 gives us 2,911.
So it is a 1919 Indian Powerplus with a engine serial of 2911…

similarly, a serial of 81M177 would be the same model and yr with a serial of 11177.

Example 3 – 86E630   <please see http://www.indianmotorbikes.com/resources/serials.htm#n 8">note 8>
Take the engine number  86E630 which breaks down to:
86 equals 7 &10+6,  think of this as 10000 (ten thousandths digit, i.e 70xxx-80xxx), then add this PLUS the 6000 (thousandths) to the 630 it gives us16,630.
The E is the year 1913. The 7 is a 7hp Hendee twin.  So it is a 1913 7hp Twin with a engine serial of 16630…

similarly, a serial of 70E433 would be the same model and yr with a serial of 433. http://www.indianmotorbikes.com/resources/serials.htm#to p">Top

4. The 1915 parts book also uses letter designation of models;
B is a twin (50 series) with smaller or lighter engine with few parts
interchangeable with other twins models C & D,
E model is the single cyl 40G series  .(thanks to Chuck Garrett)

5. Cast iron pistons std on Scout 45 unless motor bears the letter ‘A’, which uses aluminum pistons, this is from:
CGP444 on, i.e. CGP445 A.  seehttp://www.indianmotorbikes.com/resources/serials.htm#esf x"> eng# suffixes   http://www.indianmotorbikes.com/resources/serials.htm#to p">Top

6. All Export 741’s GDA101 & up were supplied with blackout lighting as standard.
1940 640B’s #’s FD01015-FDO1515 and FD02310-FD02636 were also factory fitted with optional kits offered for others.

7. The highest ‘CG’ number recorded for ’28 short frame (std) scouts by the 101 Association is:
CG2384, the highest 45″ ‘CGP” number is CGP1761, deducting 100 from each should give known numbers produced before the change over to 101.

8. All 09 thru 12 were rigid frames and all 13 were leaf spring front and swing arm rear.  In 1914 and subsequent there were also rigid frame models.
70E1005 would be the same Model 7 (61 CID Twin)  with a SN of 1005.
80E0450 would be the same engine/model with a SN of 10450 (7+1, i.e 1 in front of serial number).
(thanks <i think!> to Chuck Garrett)

9. A 1930 101 (EG901) has been found with a frame number of SI322. Originally SI344 was recorded as the starting frame number as first used in ’31. This proves frame numbers started late ’30 not ’31. (EG901 is just 24 units serials off a ’31 101). We would like to know about any lower frame numbers than SI322 please, until then we will use SI322 a starting reference.  http://www.indianmotorbikes.com/resources/serials.htm#to p">Top

10. The 28 Ace had been listed with serials starting as CA210 up, a reader has said “I was lucky enough last year to find a 100% complete 1928 Indian ACE in Sweden with all the delivery dockets and service history for the first few years, its serial number is CA196″ (thanks to Ted Worrall)

11. While not Indian (yet!) 1920-1926 Ace Motorcycle serial numbers are included here to give some insight into the numbering before Indian acquired Ace from Michigan Motors Jan ’27.
1920 A1000-
1921 A1xxx-?
1922 B3000-
1923 B3xxx-?
1924 D8000-,
1925 no production? or no info? –
1926 E1000-, (poss just E1100 up?)

12. Starting 1920 the 61ci PowerPlus (N and NE) was offered in a larger 72ci capacity named the NP and NEP (electrics) models. These 1178cc models had engine numbers starting with the following:
1920 =”RPxxx”,
1921=”SPxxx”,
1922=”TPxxx”
1923 =”VPxxx”.
These were invariably big valve models, but there was 61″ big valve models also that had an enlarged (chief size) exhaust valve. In 1923 Indian renamed the PowerPlus to Standard in order to dispel the myth that the PowerPlus had more power than the Chief. (thanks to David Bedding) http://www.indianmotorbikes.com/resources/serials.htm#to p">Top

13.The rear frame # began with 648 followed by a 3 digit #. Such as, 648 106. I used to own the bike Floyd Emde rode at Daytona in ’48. I restored it in ’78-79 and later sold it to a mid-west collector-dealer. The rear frame on these machines was standard 741 with the addition of a ferrule on either side for the seat spring bolt. To the best of my recollection they started at FDH 101 and went to FDH 150. The rear frames were stamped 648 101 to 648 150. (thanks toRoger Hensley)

14. The Sport Scout based three wheel dispatch tows (DT), (previously known as the service car in 1931 in the form of a 101 scout.  later known as traffic car in some configurations). These 1934 to 1942  where offered in many styles and had the standard sports scout engine serial nomenclature, with the following exception:  the frame number is under the seat on the casting where the tubes meet.
This configuration was revived again for the police depts in 1950-1952 in the Indian Patrol which consisted of either 45ci Scout, 80″ Chief or 30.5 ci Warrior models.. various VI members
“I had a 1951 (FEA106), a friend has FEA103 & FEA104. A FEA105 is overseas and is a two wheeler, I have pics.  You can’t say it was never made because if you wanted special orders they would make it. 103 &  were NYCFD (fire dept) bikes. I sold mine 20 years ago, my friend still has his.” (thanks to chief44312@worldnet.att.net)15. I have a listing of  actory serial number listings from 1910 to 1931. while the form doesn’t have the factory’s name on it persay,  the weight and age of the paper, the typeface and layout, plus the mention of Form M801 (Form M779 is a riders manual, M809 is a parts list and so on) leads me to believe it is authentic. Greg

16. “Starting with Motor no. EA580, machines were equipped with all steel clutch, wide transmission gears, single keyway trans shaft
and engine sprocket. prefix letters AB are used to designate this change”. from http://www.data.indianmotorbikes.com/serials /" target="_blank" rel="noopener">Form M801 . Greg

17. The Model B ‘Little Twin’ (with 42ci Hedstrom motor), continued to be sold in the UK, Australia, NZ? etc for a number of years after the 1915 cut-off that occurred in the USA.  The 1916 UK sales catalogue states:  “This model has proved so wonderfully popular during the past year, that we are continuing it on the market.”  The same model is still listed in the 1918 UK sales catalogue.
…”Earlier this year I purchased a 1917 Indian Model B ‘Little Twin’ (this is the one with the 680cc (42ci) Hedstrom motor). According to all the books that I have read there is supposed to be no such thing, as the Hedstrom’s were phased out at the end of 1915. However, I have found documentation that these bikes continued to be sold in Australia and the UK right up until 1918. I currently know of 3 such machines (1916 and 1917) and a number of engines still in existence“.
thanks to David Bedding (Beddo) from Australia  for his research and input into the early twins especially the lesser known little twin.