I encountered exactly this problem a couple of weeks ago when I fitted a NOS 1950's production SA barrel to a WW1 SMLE for a friend of a friend. The originals are the oddball EFD thread but I'm pretty certain, as MkVII said, the SA barrel inner band is a standard Britishicon Association thread, although I've already forgotten if it was 0 or 2 BA!! I'm pretty sure it was one or the other.

---------- Post added at 12:14 AM ---------- Previous post was at 12:09 AM ----------

Just went out into the workshop & checked my crib sheet. The original screw is quoted as 0.1875" dia x 33 tpi. It is stated that 2 BA will work as an alternative..