George,
The bike you are describing sounds exactly like my 62 Gran Sport. Ive
posted a few pictures of it here http://www.flickr.com/
Mine was completely original when I got less one tire... (I love the air pressure rating on the replacement John Bull tire "Keep Inflated Very Hard")! It included Campy Gran Sport derailleurs, GB brakes, stem, and bars, cottered crank, 49/46 rings, high flange Racelite hubs, and steel 27 Dunlop wheels. The frame is high tensile steel tubing , has wrap-around seat stays and it also has a light mount on the right fork (It also had the chrome heron mounting bracket you mentioned). I bought it from the original owner a few years back with only 425 miles on the Lucas speedometer. He gave me the original receipt that shows the price was $114 back in 1962.
Mine is currently in fixed-gear mode with modern wheels...sacrilege!!!!.. I know, but I'm not a big fan of steel wheels and the gearing was a little high for the streets of Seattle.
Rob Markwardt
Seattle, WA