Reply to "Headend cars on PRR mp54 commuter train consists?"

I disagree. P54 coaches were in use as the standard PRR coach, until the advent of the longer P70.
When electrification was first installed on the suburban Philadelphia line. A number of extant P54 coaches were converted to MU use. In addition new MP54 were built at the time.

Refer to Michael Bezilla’s definitive ‘Electric Traction on the Pennsylvania Railroad’.

To answer the original question. Yes B70 and BM70 would have appeared ahead of P54 in consists. As was the rule with head end equipment.