750 ('68 - '73) Starting from the original '68 Commando, finally reaching 750 MkV (from serial number 230000) in '73.
850 ('73 - '75) Mk1 - Mk3 (some Mk3s were made as late as '77).
I try to stick to Roman numerals for 750s and Arabic numerals for 850s to avoid (some) confusion.
A '74 (so 200000 series) 750 Roadster, Fastback or Hi-Rider would be considered a 750 MkIV. The 750 Interstate introduced that year is also generally referred to as a MkIV even though it was the first Interstate (so technically it was 750 Interstate MkI however, 'MkI' wasn't used for 750 models. I hope you are still following this? :wink: The list below may help to explain.
750cc
Original Commando 4/68-3/69
750 R 3/69-9/69
750S 3/69-6/70
Fastback 3/69-8/70
Fastback II 9/70-12/70
Fastback III 1/71-12/71
Fastback IV 1/72-3/73
Fastback LR (Long Range) 4/71-12/71
Fastback LR MkIV 1/72-2/73
Roadster 3/70-12/70
Roadster II 1/71-12/71
(no MkIII Roadster, apparently!)
Roadster IV 1/72-2/73
Roadster V 3/73-10/73
SS (Street Scrambler) 3/71-10/71
PR (Production Racer) 4/71-10/73
Hi-Rider 5/71-12/71
Hi-Rider IV 1/72-2/73
Hi-Rider V 3/73-10/73
Interstate 1/72-2/73
Interstate V 3/73-10/73
850cc
Roadster 1 4/73-12/73
Roadster 1A 9/73-2/74
Roadster 2/2A 1/74-2/75
Roadster 3 2/75-9/77
Interstate 1 4/73-12/73
Interstate 1A 9/73-2/74
Interstate 2/2A 1/74-2/75
Interstate 3 2/75-9/77
Hi-Rider 1 4.73-12/73
Hi-Rider 2 1/74-2/75
Hi-Rider 3 3/75-?
John Player Replica 11/73-2/75