also, some frames were sold as 531 but had only had 531 main tubes with a 525 rear.

I've weighed a few 531 frames (no fork), with the lightest being 1850g, and the heaviest, ~2200g. 531 from about 1980 and older will mostly likely be heavier than the newer stuff. 531 is lighter than much of the older Columbus steel.

I don't think the butting would make too much difference.