Correct!

My be there are some derivatives having protection diodes. But standard derivatives to me don't seem to have them. Otherwise there would be a specification about output current. 74HCMOS specifies the following:

"Output diode current (Vout < 0 or Vout > Vcc) ........... +-20mA"

This very clearly states, that 74HCMOS output HAS protection diodes, which CAN be biased up to +-20mA.

If your '51 derivative has such a specification, then protection diodes are built-in. If not, then not.

Kai