Guys:
An STC must be approved in writing for each serial/N number aircraft. The only ones not like that are the fabric STCs - they expressly give everyone permission to use their STC if they use their materials.
You cannot transfer an STC from one aircraft to another, even if you own both of them. Cannot.
The one I used to mount 8.50x6 tires is also open ended, and if Concorde had one for installing their battery I wouldn't be shocked (no pun intended).
Since we're talking spades here I'll expand a bit. The ones everyone wants to find are Olin Pash spades. About two years ago a good friend of mine handed me a set he wasn't using (plane in pieces) and furnished me with paperwork. Said paperwork was of course for a different plane. Bob is quite right, you can't just sell some parts and papers to another bloke and he now has the STC. You can't even pull them off your own plane and put them on your other own plane.
The story I heard was that Olin got tired of being poked by "the man" and stopped producing/supporting his design. I considered contacting him about transferring the STC to my plane and I even had his phone number in my hand. Then I thought he probably gets calls from dorks like me on a weekly basis and maybe I should just leave him alone.
Odd timing of this story: The spades I had sat in my hangar this entire time, and just today I shipped them (and their paperwork) back to my friend who loaned them to me.
I'm quite serious that I reverse engineered Olin's work. I thought of knocking off a set and working with a DER to get an installation approved, or possibly even develop a new STC for them. My day job has more to do with certification than burning fuel so it's not like I'm unprepared for the battle. My Citabria was used to develop two engine installations which were STC'd "back in the day", and was registered as Experimental for a while (still shows that in some FAA database searches).