Re:
Nervex Pro lugs (they're not Nervex Legere) and double fluted seat stay caps show it's a decent quality frame but these features were common to dozens of different marques in the 1950s and early 60s, and carried on right into the 80s on some frames. See my piece on Nervex lugs on the Classic Lightweights website.
With Cyclo ends, this is probably 1950s. 26.8 seat pin suggests plain gauge 531, but it could be butted, while the un-bushed rear brake bridge , if original, rules out a really high-end frame. Still a very decent frame though. Nice fork crown too.
May well be a top end Viking, probably not Rotrax (unbushed brake bridge).
The style of head lug (horns pre c55, fishtail later) could narrow down the age.
Hope this helps.