FAR 21.9, is also very important, as it mentions Technical Standard Order approvals, TSO, as an exception to PMA. Further, FAR 45.15 requires holders of Technical Standard Order approvals, as well as PMA holders, to identify their approved parts.
If it is not identified as TSO, it isn’t TSO.
Also, there are big differences between TSO parts, and non-TSO parts.
For example, the King KX-170 non-TSO radio, and the KX-170-B, TSO radio are very different.
The KX-170-B, TSO radio, while having both communication and navigation functions, utilizes a separate crystal for each function. The KX-170, non-TSO radio, while having both communication and navigation functions, utilizes a single crystal for both functions. With the KX-170, non-TSO radio, if you lose the crystal, you lose both functions! Naturally the TSO radio is more expensive.
Another example are fluid compasses made by Air-path.
These are the most widely used non-stabilized compasses used in general aviation. Air-path produces both TSO compasses and non-TSO compasses. Each TSO compass bears the TSO identification and a serial number on the compass frame under the housing. TSO compasses are made to higher standards, especially the final testing, and are therefore more expensive. The non-TSO compasses are about two-thirds the price, but they appear identical. The compass is a minimum Day-VFR piece of gear required by FAR 91.205.
If there is anything rendering the compass inoperative, the aircraft must be grounded.
If your aircraft requires, for example, an Air-path C-2200 compass, it must be TSO.
However, there are plentiful non-TSO compasses being maintained and sold by instrument repair stations who neglect to inspect the compasses for the TSO identification, required by FAR 45.15.
These compasses are illegal for installation in type certificated aircraft.