That would appear to be an approval in itself. The administrator must have had a few drinks in him that day. Just because they've made a determination that this unit is acceptable doesn't mean you can go around installing anything just because it's not on your required equipment list.
lots of wrong conceptions here. read the AC and you will see this ,
f. Standard Part. Is a part manufactured in complete compliance with an established
U.S. Government or industry-accepted specification, which includes design, manufacturing, and uniform identification requirements. The specification must include all information necessary to produce and conform to the part. The specification must be published so that any party may manufacture the part. Examples include, but are not limited to, National Aerospace Standard (NAS), Air Force/Navy (AN) Aeronautical Standard, Society of Automotive Engineers (SAE),
Aerospace Standard (AS), Military Standard (MS), etc.
Aircraft instruments are manufactured to a standard acceptable to the administrator (Aerospace Standard) (AS) they do not need a TSO, FAA-PMA or any other higher authorization. When you remove a Cessna gyro you can replace it a Century Gyro with out any higher authorization because the two Gyros are built to the same standard. As long as the replacement part meets the fit form and function of the Type design, you are good to go.
IOWs if it is a AI built to aviations standards any make will do. Keep in mind that you must meet the requirements of part 91 and the type design for redundancy. You can not remove a vac gyro, and replace it with a electronic one, because you have now lost the requirements of the type design.