you sure that's a "3-main-tube" Reynolds 531 decal?
cause my GR looks just like yours (OK, pics with follow eventually) and my decal is "All 531"...
EDIT: just looked closely and sure enough yours has that number 3, guess that clinches it.
I've assumed mine is '75...has Nitto Pearl stem and came with a straight alloy post and a "modified" Brooks Pro saddle (which I think was the one change made by the PO), same hubs but smooth-side tubular rims (no brand).