yep, 100% 531. unless someone did a good job of putting 70s era 531 stickers on the frame and fork. i doubt the frame is high-end, it does however have campy track ends.

i see what you mean about the tube mittering. i could imagine that the tube is too short and therefor excess stress is put on the lug.

...and yes, i'd be happy to get a new frame, but it would be sad to see this one go.