FAA Owner Manufactured Parts = ability to repair equipment?

I have heard of an interesting and potentially extremely useful interpretation of the owner mfg parts concession

Under CFR Article 21, Subpart A, paragraph 21.9 (5) articles may be
produced by an owner or operator for maintaining or altering his own
product. All articles which are NOT produced by the Production Certificate
or Type Certificate holder, or under FAA-PMA approval, or under TSO, a
commercial part (21.1) or are Standard part (AN, MS, NAS, etc.) must
qualify as “owner or operator produced” to be legally installed on a
type-certificated aircraft.
In order to qualify as an “owner or operator produced” article, the owner
or operator of the aircraft much have participated in controlling the
design, manufacture, or quality of the article.
That article must meet at least one of the following criteria:
1. The aircraft owner or operator provided the manufacturer of the article
with design or performance data from which to manufacture the article.
(This may occur, for instance, when a person provides an article to the
manufacturer and asks that article be duplicated.)
2. The aircraft owner or operator provided the manufacturer of the article
with materials from which to manufacturer the article.
3. The aircraft owner or operator provided the manufacturer of the article
with fabrication processes or assembly methods to be used in the
manufacture of the article.
4. The aircraft owner or operator provided the manufacturer of the article
with quality control procedures to be used in the manufacture of the
article.
5. The aircraft owner or operator supervised or participated in the
manufacture of the article. You are not considered a producer of the
article by merely ordering the article. You would not be participating in
control of the design, manufacture, or quality of the article, and
therefore the article would not be legal for installation.

It seems obvious that the aircraft owner can repair e.g. an avionics item provided that the end result is exactly the same as it was previously.

It seems obvious that replacing CMOS memory batteries doesn’t need an avionics shop, for example.

A more practical than legal issue is that you may need MMs (maintenance manuals) and/or test equipment may be needed. And MMs are very hard to find. Garmin MMs are especially hard to find and some are useless; e.g. the GNS430 one is almost totally useless. I have a huge collection of manuals which I have been collecting since I got my plane in 2002, when Air Touring failed to give me the ones I asked for (and they agreed to give me) so I realised that having them is very valuable.