Thanks a lot. T'man99
I found it on the rear drop out - opposite side to the derailleur hangar.
it's a 68334 - 3L - which suggests '97-'98. Later than I thought and catalogues from that time don't seem to align e.g. location of gears on down tube. It looks more like the 5500 in the 1992 catalogue (e.g. decals match etc..).
The campag componentry is also throwing me off scent, but that could have all been changed in later..
more digging around required.