Hi Dave,
Regarding the OB numbers you comment "that the early production did not have matching frame and engine numbers". Does this mean that later OBs did have matching numbers? Matching, or close to matching? Jack's OB has frame HF835, engine OE836X, gearbox PG844 but no history is known so I'm wondering (a) if it is likely a "matching numbers" bike and (b) can you guess at the date?
Also, rather then there being a large number of OBs produced, I wonder is there a gap in the numbers - as you comment there are lots of survivors in the 7-8-900 range. Maybe 1925 started at 700? If it helps, I have sighted (in Australia) PG gearboxes 701, 747, 806, 844, 845, 861 and OE cases 271, 739, 836, 839, [edit also 892], 943.
Thanks
Leon