Re: Missing STC and 337 found during annual
If your STC was signed off at a previous date the requirements were met at that date. BUT, you should have a copy of the STC in the paperwork.
The FAA in Oklahoma City (AFS-760) have a record of all the paperwork for your airplane and a copy can be obtained for a small fee. It was less than $5 the last time I got some. In that paperwork will be a copy of every STC that has been incorporated in your machine.
You will of course have to give them the "N" number of the machine if it has been changed to some EU registration in the meantime.