you probably have seen the '49 catalogue at Nick Kilgariff's site:
http://homepage.ntlworld.com/nkilgariff/
I bet this goes back to about that era, seems to have the "13" seat cluster and typical twin-plate style fork crown of a bronze "welded" Claud, only thing that looks unusual is the style of drop-out, not the 'Continental' or perhaps they have had the spears cut down(?)...really a beauty of a frame, in any case!
I am going out of my depth, but it seems to me that something described as "tinplate" would be tin plating over brass (or other base metal) rather than a "plate" of solid tin (which would be very soft material, softer than plain aluminum)...and over time the plating (and paint) would wear away.