That's interesting PP1. I was just looking at an old Columbus catalogue. PS tubing, 0.71 mm straight gauge, i.e. 0.028". For track riding (sprint, 6 day, pursuit) and 58+ cm frames according to the brochure. But I guess track bikes don't go through pot holes and see heavy braking. Anyway. I might have done my bilam with a 0.028 seat tube as well (not downtube), but couldn't find one in 1.125". Perhaps that's a good thing.

Also, I really like the 50s CCM Flyers. 3/8" stays as well. I'll check the wall thickness next time.