If you want chapter and verse, thing to do is phone the CAA on 01293-576161, ask for "Aircraft Certification", then when you get through to them ask to speak to the "Design Liaison Surveyor" for Piper aircraft. They should have the list of what's approved for all identical.
I shall do this tomorrow. Excellent -- thank-you.
If CAA make life difficult for you give me a shout, they're usually happy to accept my signature on such things and I'd happily swap whatever time and effort was needed for a bit of Cub time.
Kind offer -- I may well get back to you on it.
The STC if it exists will be held by the equipment supplier and they'll supply you with all the paperwork and guidance you need for a LAME to install and sign it off.
Why couldn't I just do this, then? Why does it have to be a minor mod? I didn't realise this was possible.
Both the things I want to do have F. Atlee Dodge STCs, which are very reputable. He's sort of Mr. Super Cub.
QDM