It's possible the gun was chequered post sale. Or a special request, pre sale; or returned to the factory for extra work. It was indeed made week ending 02-11-1974; by John. Part of batch B9141-9150. That week given Saturday morning was included as part of a normal working week, the gun was in batch 36 of 37. This is one of the most prolific production weeks of the MKIII; guns were made in batches of 10's for ease of recording and wages which were piece work at that time. So 370 MKIII's were made that week. To do this, Webley would have drawn in workers from other sections, as the normal production unit was 4 men.
I was head service dept technician at that time, so have some insight; but not full time contact with each department so can't be sure who would have maybe chequered it if it was in house.