Mercury gearbox

Well it likely wasn't  factory done as clearly a different set of punches were used, quite antiquated ones by the look of it, perhaps somebody didn't  want bits of different bikes getting mixed up, I have also seen people's initials, birthdays etc marked up as a form of theft deterrent.

After 128646 the P11A Ranger 750 series all had matching engine, frame and gearbox numbers. All my own P11A Ranger 750 bikes have a matching number stamped on the gearbox. After that gearbox number there is a letter M stamped. The letter N on the Mercury gearbox would denote it was meant of a Norton model. 

This Mercury was in a batch of three that got exported to Australia in October 1968. Not many others being shipped off before this date. If it is back in the UK now then, it has probably travelled more miles by sea than covered on the roads.

As the Mercury was built around the same time as the early Commando bikes the respective models  shared a good many components but each being modified to suit. So crankcases, cylinder heads, Timing Covers etc. The gear box shells and contents were similar but using different mainshafts. The Commando boxes had the  numbers stamped across the top mounting lug. See photo.