Originally Posted by
abarth
Beside the decals, is there any way to tell the stays and fork were make from 531 tubing? Special marking, unique measurement, etc....? Thanks
You'd need a way to measure the tube wall thickness. Many bikes used lesser tubing on the stays and forks especially. If you know the bike model, you can do research to find if it was a full 531 or just the three main tubes.
I think the larger sized bikes tended to use a beefier tubing on the stays and forks; at least for some builders.
Columbus had a "tre tubi" sticker to denote the three main tubes being SL or whatever.