Tubeset Question

Got an old bike with 531 stickers on the downtube and fork. Having stripped the paint, the Reynolds spec is visible on the toptube:

"Reynolds 26.0 x 0.7 / 0.5D 653"

Anyone know what the tubeset is likely to be? 531, 653, something else?

Stickers can be misleading....the writing being on the TT would imply 653....however...I have a 653 frame and it takes a 27.2mm seat post....So perhaps you need to check the 26mm diameter and is compatibility with 653.....as 753 comes up as either 27.2mm or 27mm.