We are exactly on the same page.
However, I don't think the OP wants to "manufacture", or certify, a part.
As I understand the OP's question, he has a component, installed under an stc, ie not original, OEM installation.
Let's assume, for the sake of argument, the equipment is a "Whiz-Bang Navmaster 500" PFD, p/n 500-1; a cutting edge product of the giant "Navmaster corp". This model & p/n Navmaster PFD is original equipment, and supported by an OEM (and the vendor, Navmaster) in several models of the oem's Certificated, current production aircraft.
A large avionics shop, "Peckerwood Avionics" sees the popularity & usefulness of the new "Whiz-Bang Navmaster 500", and approaches the vendor (Navmaster corp) for ten ship sets of TSO'ed PDF's, p/n 500-1. Business is business, after all, so the deal is closed. "Peckerwood" puts it's guys to work, for a STCed installation on a "Belchfire Flite-King" a popular, but out of production aircraft. They see a market for replacing the "steam gauges" in this fleet of 5000 "still on the books" aircraft. Peckerwood's engineering dept makes up the drawings, their on staff DER signs off on the changes to the Instrument Panel, it is approved by the Feds, and an STC issued. They sell eight STC'ed installations, and are preparing to order ten more shipsets.........
Then Navmaster issues a mandatory service bulletin. There have been "Ghost images" when operating in the night vision mode, and false altitude readings. They send out SIM cards with software updates,to all operators, but in 15% of the fleet, the errors remain. One aircraft, an OEM installation, has a near-miss due to bad altitude information. The OEM, who has aircraft in production, insists the vendor fix the problem. Lot's of bad publicity affecting sales. The vendor, now decides to recall all units from the field, upgrade some hardware as well as software, re flash the chips, whatever; & rigoriously test the units, and send them back under a new P/N, 500-3........
In the meantime, another Navmaster equipped aircraft, while busting minimums takes out the approach lights at BMF, in a "reportable accident" The NTSB & the FAA are now involved, and after being satisfied Navmaster is on the right track, issue an AD note, requiring the "-3"
Meanwhile times are bad at "Peckerwood" avionics, the crash of '09 has hit them hard. They have laid off half their staff, and worse, they are on COD with the Navmaster Corp!!
The operators with the "Peckerwood STC" are left out in the cold. They can't even pay out-of-pocket for a p/n -3, and legally install it!! The AD note is very specific!
The operators all get together, (one of the operators has a "Blog", "Peckerwood STC sucks") and they retain council, who sues "Peckerwood Avionics".
To Late!! They have declared bankruptcy, and the disgruntled, laid off employees have stolen all the shop equipment and supplies!
And that's why I think the AMOC letter from the Feds, would be a good idea.
Happy New Year, everyone!!