On Tuesday, July 28, 2020 at 6:53:07 AM UTC-7, Charles Longley wrote:
Sorry I wasn’t clear. A FAA form 337 is required. A field approval is not required.
I live near Seattle if anyone needs a hook installed.

Charles, thanks for explaining this. Its a puzzle to me that the FAA does not encourage, or even mandate, inverting all Schweizer towhooks. The FAA show an inverted Schweizer towhook in their FAA Glider Pilot Manual (albeit they show a non-approved ring) and the FAA also note the danger of the non-inverted Schweizer installation in their AC-43.13-2B. The danger of the non-inverted installation is also commonly agreed among all tow pilots I have spoken to. Yet the FAA don't encourage changing all such hooks, but rather encourage leaving them non-inverted by leaving paperwork obstacles in place.