As I recall there were two different designs of Nervex Pro lugs - Mk1 and Mk2.
The Mk1 resembled the later Mk2 in every detail except for the little twirl on the head lugs. The Mk! version did not have that pretty "curved -x type" pattern, but ib=nstead had two short sharp prongs, a bit like a refined versions of some prongs on some Serie Legere lugs.
Certainly the Mk1 version was available in 1951..and the Mk2 came not much later. I have a Hill Special and an incredibly light Mercian Pro, from that era, both of which sport the Mk1 lugs.
Norris Lockley, Settle UK..where the sun has not shone, nor the sky been blue for weeks on end