Hmm, a fascinating question. I have a '39 Bluejacket's Manual, and a '40 Knight's--but, both are in deep storage and not something I can bring to hand.
Now, if memory serves, PG referred to the Panay-type gunboats. If a distinction were needed for a topedoless Peter Tare, it might (only might) be PGB for Patrol Gun Boat.
It's good to remember that may have been one Navy Department, that there were two Navies--LantFlt and PacFlt. Atlantic Fleet personnel were very much influenced by the Royal Navy. Enough that LantFlt types would refer to the PTs as MTB, even though that was not the by-the-book designation. But, it probably simplified chin-wags over Singapore Slings.