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I have to disagree on the lugs, every detail of them matches the nervex pro with horns. Imho the bottom doesn't look broader than on other examples and they look completely different from the oscar egg luggs especially at the transition to the TT and DT. There are a few falcon road san remo road bikes from the 60s with nervex pro lugs so I think it's likely that they were also used on some track models. I don't see why they would have taken another lugset and replicated the exact look of nervex pro, especially since the san remo was the top of the line model and money probably didn't play that much of a role. In the below photo you see the comparison with a schwinn that used nervex lugs.
Yes, sprints are the same size as 700c.
Still convinced this is a 50s frame, I've had a quick look at a couple of Ernie Clements bikes. Check this one: http://www.classiclightweights.co.uk/bikes/ernie-clements-browne-rb.html
I'm still feeling yours aren't early Nervex lugs. They might be cut down Oscar Egg Super Champion, however. The horns are too fat and the flat bottom looks a bit too broad to be Nervex but it's really close to the shape of an uncut Oscar Egg. Fancy cut lugs was what you wanted in the 50s. See the frame number on that one is quite close to yours too and is dated 1953.