UK Parts list number 4 dated Sept. 10 1977 identifies 3 different part numbers for the #1 and #4 exhausts, and 2 part numbers for the #2 and #3 exhausts. The third type (#1 and #4 only) is indeed due to the change in length of the mounting bolt which occurred at VIN 1001222.
Technically of course there is fourth type, part number ending 620, as fitted to machines originally supplied to a number of European markets (different diffuser fitted).
Chris