Originally Posted by
Dirtdrop
I don't know the answer to that question and I'll bet you don't either. I don't know why they couldn't roll the tubes just as thin back then. The process had been developed about 40 years earlier.
My 1959 Carlton takes a 27.2 mm seatpost. That means that the walls are .7 mm thick. Not a lot of modern steel bikes have thinner walls than that. That's five decades.
Your Carlton was built with 0.7/0.5/0.7 or possibly 0.7/0.4/0.7 tubing walls if it's made of 531 db tubing or 0.7 mm straight wall tubing depending on the model and frame size.
The really thin wall Cr-Mo tubes came out when "oversized" tubing began to replace "standard diameter" steel tubes. This happened as both ultra-high strength heat treated alloys became available and steel tried to compete with Ti and Al for building light frames. Some of these had butted sections with 0.3 or even 0.2 mm wall thicknesses.