You say that parts off your 51 and the 55 that he has are interchangable, right? You could tell him that you broke a part on your 51 and if he isn't using the 55 you need to get a part off of it. You could also tell him that you need to get a part for the 51 and it it so far gone you need to see the 55 to make sure of the part or parts that you are going to be ordering are correct and that nothing else is broken that are near the part you need.