The problem with your logic is that a 40+ year old transmission may have changed hands several times before it may or may not have been stolen. As such, without the serial number the correct owner may not know whether the transmission for his car is the one you have. If you were to post the serial number, putting the right owner with the transmission would be much easier and I bet the owner of the car this transmission is meant for would gladly pay you at or above a market price to reunite the two.