Importing car parts to the US - proof of purchase

[3xpendable]

I have some car parts that a friend of mine wishes to take back with him when he returns to the US later this year (2 gearboxes he wants to take back with his car in a container) However we have looked up the requirements and it appears that for customs paperwork of car parts you must have proof of purchase.

Herein lies the issue. I am happy to give them to him as they are of little value to be but the issue is I took them out of 2 scrap cars probably 10 years ago so have no proof of purchase. What can we do?

[3xpendable]

I thought of that, just wan't sure if it had to be some sort of official reciept.