I still find it difficult to grok the regs, and the more I read about STCs and PMAs, the less I know. But if it helps anyone else, I
think the following enables us to use a Concorde sealed type RG24-15M battery:
https://concordebattery.com/otherpdf/finalfaapma.pdf
And how, oh fellow novices you ask, do we know this is a sealed type battery? We don't. But your (or at least my) IA does ... assuming it's even true. The Concorde site gives us a clue on its home page with mention of the RG series under that description. My assurance went pear- shaped when my IA seemed to suggest however that an RG24-16 was not. Now, another question (or two, or three) you might ask:
- why isn't the RG24-15M listed for us on the web pages?
- what's this "M" designation all about?
- and most importantly, why does the above-linked pdf appear to list our aircraft under Cessna Singles?
Well I intend to call them soon to get those answers.
And thanks once again for the helpful replies that got me this far. I need now to work out what modification(s) I need to do to install such a boxless battery, and how to keep my IA out of trouble. He believes the original box provides some structural strength as installed, and that without it the bracket should be doubled with additional material. He's also not sure about the strap-down method, or what approval (i.e. paperwork, authorizations, etc) will be needed, if any. I'll follow up on this as well.