2-Piper:

"Personally I think the Purdey double bolt is a superior design."

Care to elaborate? The three guns I have come across with round underbolts are the Mod21, Horsley and a Belgian Darry. The underbolts were in the 9mm diameter range, offering 60 square mm against shear forces. The Purdey underbolt offers slightly less cross sectional area in most English style guns.

Additionally the round bolt seems easier to drill, fit and finish at the manufacturing and repair phases.