Hi sorry not to have replied sooner, sometimes off forum for a while. But looking at your MKIII: as usual with ''A'' prefix and in .22 cal. it crops up twice, as I've said before Webley ran two series of rifles with the same prefix letter one after the other ???!!!
OPTION ONE is block 5 of 6 [guns made in blocks of ten] W/E 25/10/1969 this has no small letter shown in the book but that's not unusual. Given the week ended on Saturday, I would say Friday of that week. ''J'' was usually the letter associated with Joe who was in fact the leading bench hand. Webley at this time used roving inspectors across the factory as well as having a dedicated ''test'' room; I am not aware of inspectors at this time putting an I/D mark on airguns as was common practice in the 50's and early 60's.
OPTION TWO is block 1 of 4 W/E 22/7/1972 this is marked in the book as made by Dave, so if the gun had a small letter I would expect it to be ''D''. Given there is not enough production to cover the whole week, but production almost always began on a Monday, if there was a later start it was often shown as such I would accept this gun was made on Monday of that week.
With the information available it is reasonable to accept your gun was made as option one.
Hope this helps
abellringer