The original FLM career plot was designed around the TSR2 and the theory was multi skilled FLMs would pump gas, wipe windows etc., AND fit bombs. Any Rects would be done by Split brain Heavies and Fairies (the old appos). All this would be overseen by Supertechs (5 trade wunderkind GUARANTEED to be Chief Techs by age 25). When the TSR2 was cancelled, the system that the rest of the techy used world obviously took over. (Mechs and Fitters with pretty much what we have today hierarchically).
The Trades review of 1968 I believe, kept FLMs and Appos as a good thing, but stopped recruiting Supertechs; and fortunately (and there has to be some plumber out there with horror stories) removed the Wpns trade from the ones left. Nice blokes supertechs, brains the size of a planet but no fecking use as engineers!
When I joined in '78, us FLM's were made to choose a trade within six months of becoming an LAC, and when/if selected, you went and did a mechs course straight into a Fitters course. Or if you were after the decision to disband the FLM trade, an assimilation course then you posted back to your unit to wait for FT.
All that said, My first mentor as a FLM was coming to the end of his engagement having spent 22 years as an SAC! Happiest bloke I ever met, last heard of in British Transport police, though he will be very retired now.