At the risk of nitpicking, I believe "BAU15S" refers only to the base (as "E26" refers to a standard household light bulb base),
so there could be multiple bulbs that would fit into the socket, but they may not be the right bulb due to color, dimension, voltage,
or wattage. That said, there do not appear to be too many unsuitable BAU15S base bulbs, but there are some.
The right bulb for an AP1 rear turn signal appears to be a PY21W (Phillips) or 7507 (Sylvania Osram), both of
which have a BAU15S base. The 1156A bulb mentioned has the slightly different BA15S base and can apparently be
forced in and/or modified to fit.
AP2s appear to use a different bulb (7440) with a totally different base.
Here's a chart of some light bulb bases for anyone curious.

 

AP1 HONDA S2000
Rear Turn Signal is not a 1156 bulb size.
Its a European bulb size which is a 7507, the prongs are not straight, but people have decided to modify their 1156 bulbs to fit a 7507 housing. I just wanted to put this here because I've been using this list and wasted my time buying a 1156 instead of a 7507 which is a direct fit. Sure I could've modified my 1156 but I didn't buy a bulb to modify it.