Darryl I agree with you that you can use a pairing that has been approved by a STC, TC or ATC on another aircraft for approval basis on an aircraft that is not on an AML per the MOU. I used the MOU to put a Stratus transponder in a Pawnee that isn’t on the AML. What I don’t agree with is the term “informational 337.”

If you review the two documents I put a link up for you’ll see that there is several avenues for finding approved data. As an IA I can’t approve data. What I can do is determine if data is approved by someone else. (STC, DER, 43.13, MOU, etc) I then sign a 337 to attest to that fact. The 337 then gets sent to the nice folks at FAA registration. Where it gets put in the aircrafts records and pretty much never sees the light of day until the airplane crashes or someone buys it. My ASI is typically never involved unless I have a question. Years ago (before I became an IA) they had to approve every 337 but they got to busy with the airlines. The onus is now on the lowly IA.

I hope this makes my position clear.