On Wednesday, May 9, 2018 at 2:05:51 PM UTC-7, Darryl Ramm wrote:

This is going down the entirely wrong rat hole.

I was going to say "Looking up a dead horses ass". Granted, there are different interpretations and basis for installations. I once had an AI refuse to sign off an annual because of an S80 until I produced a Factory Service Letter outlining the installation of non TSOd equipment. On my last install (Different glider) I went with the basis outlined in the Maintenance Manual (Which allowed for the installation of ANY GPS). When I did a transponder install a couple of older mechanics said it would involve a 337 but the FAA said this is considered a minor alteration (And has been for some time). A few times, When I was feeling lucky, I talked to the FAA and went all the way up to the Regional Manager for Certification for the Northwest US. This person dispelled a lot of Internet Folklore.
To the OP, just ask around and possibly check with the manufacturer.