When I get home I will have a look in the "dungeon".

Are you talking about "original" (long) Lee type, Lee Speeds? The thread is the same as a SMLE, but the "Knox" form is quite different.

That is part of the catch: even your "basic" SMLE barrel has several features that are "indexed: the extractor slot, driving flat on the Knox form and the keyway for the frontsight band.

The earlier "long" barrels are a bit more complex at the "fat" end.

If you are doing your own machining, it is a basic measuring job on an "old" barrel or the the receiver itself, to find the required length.