I have not seen that shifter braze-on before. I have a few 50s frames. One has no shifter bosses, and the braze-ons on the other two look like this, which I believe is by Cyclo, and is for a Cyclo-Benelux shifter:






Obviously yours is different. Is the stop part shown (for lever and cable) integrated, or removable. Also, do you have them on both left and right (the top example here does) or only on the right side (bottom example)?

As to dating: the head tube Nervex Pro lugs are (according to Mark Bulgier, who seems to know his stuff about this) the second version, where the terminal on the midline of the head tube is a "whale tail" rather than the first version's "vampire fangs". I got this from the internet-BOB Google Group:
https://groups.google.com/forum/#!searchin/internet-bob/Bulgier$20Nervex$20whale$20tail$20vampire/internet-bob/7LOE0cCuq-M/RrvyXCrRAQAJ about the 6th post down.
You can see that the lugs on my top example (an F.W. Evans) are the same type. I don't know when the 2nd version came out, but production of the 1st seems to have petered out in the early 50s.

Your point about the lightness of the frame is consistent with my experience: the early 50s frames I own (R.O. Harrison 1951 and JRJ [later Bob Jackson] date not sure, but definitely 50s) are the lightest ones I have in a comparable size. It's those pencil-thin stays, and lug thinning! And scrupulous construction, I think.

One possible reason that the frame doesn't take a 27.2 mm seat post is that the seat tube might be double-butted; in Reynolds 531, the seat post was typically butted only on the bottom end.