type of finish on L112's

[mab_001]

Have rebuilt a pair of L112's and want to touch-up the finish. I know that the veneer is American Black Walnut, but can anyone tell me what type of oil was used when they were finished - linseed, tung, etc...? Raw, polymerized...?

This is one area where I don't want to use any trial & error.

Thanks,
Mike B