I don't know the answer directly, but I do have a couple of suggestions:
1) Go to this site and download the entire manual. The manual contains directions for resetting the code:
This site requires you to create an account to download the manual, but it is free and only takes a minute.
2) As a truly last resort, if you have a bit of time on your hands, enter the possible codes sequentially until you hit the right one. Seems like a lot, but from experience on most of these you can enter 6-10 a minute without trying very hard, so if you had to enter every code (statistically almost impossible) it would take you 16-20 hours. I'd actually make a short list of likely codes first (0000,1111,2222, 1234, family birth-dates, last 4 digits of phone numbers, street numbers, etc. from the family that owned it before; it's almost unknown that a truly random number is used, since no one can remember it!). I know from experience that it can be done, but I'd try the manual first!
Good luck, and please let me know if this helps. Also, a rating would be appreciated if you deem it appropriate!