They are production identification stamps.
If you ever decide to polish a set of cases you will find lots of them.
Parts were inspected using go / no go gauges and the gauge size got stamped into the part
Then there were batch numbers because BSA did not use modern "just in time" production planning.
Thus a gearbox casting could have been made anything up to 3 years before it was finally fitted to a bike.
So if for instance the 20 B was a gauge number , all of the 20 B's would go into a stillage till there was enough of them to warrant final assembly using the "B" set of shims on the "B" shafts .
Parts were mostly machined on production machinery using production workers.
The machines were phnumatically controlled so worked between stops.
The operator would take samples if it was a high volume production or check every one against a set of gauges.
They would either be tosses into stilages according to the finished size or when they got to a certain gauge the operator stopped the machine and pressed a bell alerting the tool setters to come & readjust the machine, change tools etc etc
There were no tradespeople fastidiously measuring then machining then measuring & machining as you or I would do today except in the competition shop , experimental shop rectification shop & other such departments .
BSA pioneered this method of production of motorcycles and cars starting back in 1926 with the round tank that was the first model designed to be built with fully unskilled ( well not trade qualified ) labour .
Th tradespeople made the tools and the gauges that the production workers used, not the actual motorcycles with the exception of the preproduction bikes including the trade bikes & some of the competition bikes .
