Originally Posted by
unworthy1
It's been a long time, but IIRC they were on at least the TT and DT (maybe not on the ST, I can't recall) and they were near the headlugs. I believe that nearly all 531 tubes came stamped like this, but often get cut off on smaller frames since they are near the end(s), on the butted section.
All the tubes were stamped, but as you note the stampings on the main tubes were often lost when the tubes were trimmed to size. The usual places to find them on such frames is the bottom bracket end of the chains stays and the steer tube above the fork crown:
