It is a remarkable coincidence that all of these types, RRC 292, RRC 335, and RRC 384, all made by different moneyers from different families each represent a voting ballot* with the letter P. I find it hard to accept that in each case the P stands for a tribe. Why would they all select the same tribe or initial if it is indeed generic? Compare the voting tablets market V for V[ti Rogas], RRC 413 and also as a controlmark for Piso Frugi (obv. 33, Crawford 1974: table XLII), or the A[bsolvo ] C[ondemno] of RRC 428. I think the P needs a bit more investigation.
* – On the Nerva coin the P on the tablet is more commonly understood as a placard identifying the unit presently voting at the polling station.