If it is a barrel-type lock, the key code is usually stamped on one of the flat sides of the lock cylinder. Open the panel so you can see the back of the keyswitch. You may have to remove the rotary switch and the hex nut. Typical codes would be 513, 514, 515, etc.
If it were a custom-ordered barrel-type lock, the code will not be stamped. The factory stocks keys up to key code EX 529, and you may want to order some keys to try in it. There may be other numbers on the back of the panel that would help to identify the parts. Contact a Customer Service representative for assistance.