I am trying to find the knives for my father-in-laws 6" Rockwell Jointer, it's approximately 30 yrs. old. I have tried Porter Cable, no luck. The model # is 37-608, Ref.#-108, Part #-37670, Serial #-81E03347. It's a set of two knives. Is there knives from a different model that would work? Any help on this issue would be greatly appreciated!