I'm going to stick my neck out here and politely dissent from juvela 's ID of the lugset as being by Nervex. The Nervex lugsets typically have somewhat thicker shore lines on most bike frames I see that use them, and if they're thinned they show more taper; also, offhand I can't easily correlate any of them on the chart above with those on the Stan. Allin. R.O. Harrison was one builder who fashioned his own lugs out of (noticeably thinner) sheet metal, and varied the profiles quite a bit from build to build -- I will speculate that this bike is another example of that. The lug profiles on this bike are both simpler and less "rote" than Nervex examples we're all familiar with. Besides, if it were a Nervex lugset, then why not a Nervex bottom bracket too (which it doesn't appear to have)? And I’m fairly certain that, rather than Nervex 1008, the rear dropouts are "Stallard" pattern ones, which may have been manufactured by Cyclo, used commonly on British frames until the mid-1950s, when Simplex and Campagnolo became more widely used in Britain. Ref. the top figures here:

My guess is that this frame dates from the late 40s or early 50s. I could be wrong (I admit that I'm nowhere near being in juvela 's league bike-lore-wise), but I would ignore the MAFAC brakes, as a later addition. The Reynolds tubing decal on the seat tube, with the apostrophes on the rotated '531' overlapped by the "with" and "butted" (and no R-in-circle yet) was in use, at least according to the decal purveyor Lloyd, from 1938 to 1953 -- and you can be pretty sure that's native to the frame.


There's a later version with apostrophes, but the text on that one doesn't overlap the '531'. Just sayin'.