Re: Missing STC and 337 found during annual
What I do know is that an STC belongs to its owner and requires the owner's permission before it can be used.
Some charge for this, others don't. You will need to contact John Jewell Aircraft and ask them nicely.
The STC'd part is then fitted under a 337, probably. If it's a major mod it may need DER approval; most UK FAA outfits use an American outfit called DER Services for that. This is an area where somebody wanting to make money out of you could become very anally retentive.
I hope I got this right. I have done this only once myself. Haven't you got an engineer working on this job who is familiar with FAA paperwork procedures? PM me if you need a good contact.