525
As far as I was aware, 525 and 520 were tube sets made by Reynolds specifically for seat and chain stays. They were designed to be built up with 631 653 and 825 831 and 853 main tubes. I believe 520 and 525 are not made in main tube sizes could the confusion be witrh 825 as GT made quite a few frames from that, same properties as 853, i.e stronger when heated but just a lower tensile strength. i also would have theough to be badged Reynolds it had to be Reynolds. I once had a Bravado Team Issue in 825 that had belonged to a GT sponsered female rider.