Originally Posted by T-Mar

Not quite correct, but close. According to a 1988 Bicycling article;

"Reynolds 653 is a composite tubeset that uses 531 forks for shock dampening and a 531 main triangle that's slightly work hardened to increase it's strength. The rear triangle is heat treated, but to a lesser hardness than 753, The result is a tubeset that costs the builder about 1/2 as much as 753 and can be brass-brazed."

The reason for referring to the main tubes as being 531 based are because 531SL used these tube gauges long before 753. When originally introduced, 753 was only available in metric sizes and used a thinner (28 gauge/0.35 mm) centre section. The tubing thickness in the above chart in post #4 are for the revised, imperial sized 753R tubesets introduced circa 1983-1984. The R suffix was dropped later in the 1980s.