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. This particular Follis was a 25" frame (and too big for me).