I forgot to put the engravings were another reason for picking it up in the 1st post - the bars have a design engraved on them! According to Hilary Stone "GB stood for Gerry Burgess founder and owner of GB Cycle Components." I'm slowly finding more info as I go.
One of the reasons I put this bike into the 70s was because the Shimano hubs are stamped "Y" which is s'posed to be 1974.