Ben Miller sez:

You can't just guess at this. As sue suggested, you need to look at the
circuitry and figure out what that diode function is. ...

OK, here goes:
German-made paper-handling machine, c. 1989. Circuit activates solenoid,
taking several inputs from other sensors, signals, etc.

Anode of unknown diode is to ground, cathode to drain of BUZ72 MOSFET. 1uF
cap also from drain to ground. Drain connects to solenoid. Drive voltage is
42vdc.

Thanks,
--
John English