HP 35s Rom Version

Press [C] and [XEQ] simultaneously to go into self-test mode.

Then press the + key nine times to step through all the HP 35s self-tests (pressing + will let you skip the keyboard test).

The top line should display "35S-OK", and one of the flag indicators should be set. Add 1 to the flag that is displayed, and that´s the ROM revision.

When you´re done, press [C] and [GTO] together to leave the self test mode.