Reversing Key using pattern

I have a device that uses feature codes to turn on/off features. It is system serial# specific.

I have a list of valid codes based on serial#'s... so I thought I'd see if I could find a pattern. Well I did for the one I tried. And I can create valid keys for that feature. However I would like to try to figure out the algorithm as there are over 30 features and that would take forever to manually decipher. This is probably more in line with cryptography than cracking.

Does anyone have information that may help me discover the algorithm based on pattern recognition? The license keys are 12 chars only from back in the early 1990's.

Thanks,

P.S. My guess is that they are using a lookup table. If this number = x then lookup and use k, etc. But not 100%. Program is not MS-DOS/WIN based so no debugger.