Yes, there is supposed to be a VIN on the toploader HP transmissions. However, some, or many, or all (a debate rages) Metuchen built HP Mustangs seem to not have a VIN stamped transmission (engines are included in the debate). If it is stamped, it should be on the bottom of the transmission. This includes HEH BX and HEX T boxes, the latter "T" being found after the change over date. The C4 did not get a VIN stamp.
Jim