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September 2018
Frame information introduction:
While my original research on this subject was somewhat narrow in focus, it's use over time has widened significantly. Different country, region and state regulations have differing nomenclature and requirement for items (nortons) to be identified faciliting registration for road use. The specific items of identification on a norton had evolved over time and the items are fairly clear cut. There use by individuals and governments often are ignorant of the norton identification markings. Nortons VIN (vehicle identification number) style started in the early 1900's. Frames were stamped with VIN numbers directly in the metal frame including the earliest commando's. The VIN numbers grew to six digits.
Aluminum commando VIN plates:
Then the norton VIN was placed on an aluminum plate with hammer screws attaching it to the frame neck. Unfortunately this type is ripe for fraudlent manipulation, whether innocent ignorance or criminally motivated.
Frame Batch Sequence number (my term):
For 1973-on the commando added a norton "Frame Batch Sequence" number (my term). The FBN is often very hard to detect being stamped in a verticle line on the right hand side of the frame neck, and covered/buried in paint. Some government agencies will choose to use the FBS in lieu of the long standing norton VIN system. This numbering system is NOT consistant start to finish, probably caused by the production termination of a failing company.
In america the frame end item is typically used as the vehicle identification for legal purposes. However the usual matching engine and transmission numbers may not match the true frame/machine VIN. This may or may not cause legal complication depending on jurisdiction in the USA or in other countries. Innocent or fraudulant remarking and misidentification often causes documentation problems. The passage of time and transactions increases the occurance of these issues.
Frame manufacture origin:
It is my belief that the later frames with wide or narrow loop came out of the difference from British Renolds or Italian Verluchi supplying British imperial or Italian metric dimensioned tubing. I have not found any FBS marking system that identify the source of the frame. THE ONLY possible identifier, which I currently believe, is whether the VIN tag is mounted on the right or left side of the neck. This aspect is still under investigation and research.
The 850 Commando Frame # Mystery
Dave Comeau  Jan 2001 Updated  25 Feb 2007

In pursuit of information concerning the fitment of a fastback tail section on late model frames with wide loops. I have tried to find a pattern, but so far have been unsucessful. There are both wide and narrow loop late frames. The Italian frames are easy to detect with the small frame tubes being 25mm and the large backbone at 60mm. The British frames are imperial sized tubing 1" and 2-1/4" backbone.

1. VIN- the traditional Norton VIN number on the aluminum plate, engine and transmission note on vin 30xxxx are 1973 31xxxx are 1974 325xxx are 1975-end
2. MFG date stamped on the plate.
3. Frame Batch Sequence number (my term)    #850#F.........#     #=circle with 2 horizontal bars.
4.  Rear frame loop width. Normal 6 3/4" (will accept fastback tailsection), Wide 7 3/8". Measured between the frame rail tubes 4" forward of the rear of the rear frame loop.
5. Letter size as stamped on the aluminum plate and frame batch sequence # . Small 5/32" or big 1/4"(like on engine).
6. Dash or No Dash in frame part number located between the 06 and the 4140 (73), 5404 (74) and 5632 (75E-Start) on the front of the left front tank mount.
7. The aluminum VIN plate is mounted more to  the Left or Right side of the frame neck.
8. The coil bracket mount hole on the tank mount are ROUND or short "slots" with a rounded front and rear ends.

VIN MFG DATE Frame Batch sequence # LOOP N/W S/B dash/nodash L/R rd/slot
302591 Apr-73 #850#F 00108# W        
309911 Nov-73 #850#F102329# W        
313761 Mar-74 #850#F106144# W sm nd R rd?
317462 Jul-74 #850#F107671# N sm d L rd
325545 Nov-74 #850#F125479# ?        
326154 #850#F125517#          
326209 Jan-75 #850#F125681# N sm d L rd
327522 Feb-75 #850#F128153# W        
328348 Feb-75 #850#F128888# W        
329952 Mar-75 #850#F130588# W sm nd R sl
330046 Mar-75 #850#F130370# N sm d L  
330419 Apr-75 #850#F130759# N sm nd R sl
332473 May-75 #850#F332473# N big d L rd
333093 Jun-75 #850#333093#  "no F" W big   R rd

*Starting in may 1975, It appears the VIN was adopted to complete the frame#
If you have any VIN/data contributions for the chart please email me.