Posted too fast:

"No4 Threads are DEFINITELY Whitworth form." should read "No4 Threads are DEFINITELY NOT Whitworth form except for that 1/4"BSF job

Going with "general engineering" threads made sense when involving thousands of subcontractors in wartime production. Managing the "Peddled Scheme" for SMLEs in WW1 must have been a nightmare to provide all the gauging right down to simple thread level. Specialised tooling would have been nececessary to make even the most basic screws in the rifle. How many basic lathes can cut 37TPI without a lot of fiddling in the gear train? However, every man and his dog would have had a set of BA tools and gauges. These days, CNC lathes can easily make any weird thread pitch you like; it is sometimes a struggle to get the operators to actually grind the correct form tool.