Kurt, I don't know the answer to that. I'm pretty sure the P13-9s, which were the only '72 model that the Campy's were an option on, had the brake bridge mounted lower on the seat stays than the P15-9s (remember, the P15s needed fender clearance). So, the Campy rear brakes may have fit OK without a drop bolt. I do suspect anyone trying to mount the early Campy side-pulls on a P15-9 would require a drop bolt because of the higher brake bridge.

There's also the question of whether P13-9 frames built for 27" clincher rims had the brake bridge higher than those built for tubular rims, and I don't know the answer to that one, either.