My '72 P15-9 was originally equipped with tubulars and I relaced the hubs to 700c clincher rims. Since these rims have the same 622 mm diameter, the stock Weinmann center-pulls that came on the bike from the factory worked fine. The front is 610, the rear is 750.

Since the pads are about as far down in the slots as they will go, I believe the same brakes would work on 27" (630 mm BSD) rims as it looks like there's more than 4 mm upward adjustment in the caliper brake pad mounting slots.