Wanted to add my 2 cents to this thread since it's one of the first ones that comes up on a Google search for SH775AA. I've been testing a bunch of these Shindengen regulators after retrofitting them to a number of bikes, and the difference I've managed to find so far is that there's a different voltage set point for each: (Note, these were all tested on the same day, same bike, same battery, same wiring, etc.)
SH775BA: 14.2V from 2000rpm to redline (3 samples tested, one brand new, two used - all identical)
SH775AA: 14.7V from 2000rpm to redline (2 samples tested, one brand new, one used - both identical)
Granted, I have a fairly limited sample set, but the trend is obvious and it makes sense given that the Shindengen part numbers are different. Shindengen does also note on their website that they will make these units custom for any OEM with whatever regulated voltage they want.
Your mileage may vary of course, as different bikes will have different voltage drops through the wiring, connectors, etc., but in particular if you're wanting to use a LiFePO4 (Lithium) battery in your bike, you might be better served getting the SH775BA - a little less voltage means a little less stress on the cells.
Nice work. Did you measure this at the R/R terminals? That would eliminate wiring differences.