- does the fact that the ejector screw is absent from this n° 4 receiver show that it was never actually used on a Lee-Enfield service rifle during WW2?
Redundant query! WW2 was long over in 1951, when the receiver was made. The No.8s were built using No.4 MK1 action bodies in the 1950s. See "the Lee Enfield Story" P.442. But I imagine the Mk1 bodies were probably - at least partially! - new but surplus factory stock, as the No4 Mk2 was generally in production.
- was it common for .22 LR Enfield trainers to undergo FTRs?
Most definitely not. I have collected auction sales adverts of (almost) all No.8s offered here in Germanyfor several years. I have never seen one marked with FTR before. The FTR UE 66 refers to a factory thorough repair at Enfield in the 1960s. See "the Lee Enfield Story" P.442.
BTW. thanks to the FTR, that looks like a "very low mileage" No.8. You were lucky!
Patrick