P1 has port P1S0-5 “SPARE0-5”… these are additional gpio with particle firmware support.
See last sheet of Pin Out Diagram.
Here’s a thread discussing firmware support.
IMHO, that’s the big sell for why anyone would want to header-proto with a P1. If the pinouts are going to be the same as a Photon, then might as well just put down a Photon (+mem) and be done with it.