Just to add another possibility to the mix, I acquired a pre-loved custom GC a few years back. The original spec included 'ES-NC' ($400 extra!)- the 'NC' is short for 'No Controls'. This means that there is no exterior sign of electrics, apart from the jack socket. It also means the ES board and battery holder are situated inside the guitar, and accessible through the sound-hole.
While your situation is more likely to be a retro-fit pickup, it is possible that it is an 'NC' version of Taylor's ES system. It will probably be obvious once you get a look inside, but, if it's not and you need to make sure, an email to Taylor with the serial number, asking the question, should result in a prompt reply, one way or the other.
I must say, I find it a real pain to have to loosen the strings every time I need to change the battery - certainly wouldn't want to do it on-stage!
As an aside, does anyone else find that batteries seem to last a lot longer with the new, improved battery holders?
