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.