Reg/Rec means Regulator/Rectifier Module. it converts the AC 3 Phase energy produced by the generator (stator) to DC voltage, and the Regulator regulates the Voltage to less than 14 VDC.
PO = Previous owner.
You'll find the R/R (also stands for Regulator/Rectifier) in plain site on the left side of the bike, under the mid panel (the one with the huge hole that you can see the fan in). doesn't matter right now though, i know you have a bad design R/R. you'll need to get that replaced if you plan on riding at high engine RPM at low speed. i'm talking 30 mph in 3rd gear. that's too high. even 4th gear @ 30 is kinda boarder line. if you'd like me to explain more in detail about the problem with the R/R i'd be more than happy to. it gets a little in depth, but it's far better (and cheaper, since you could get the seller to take it off the sale price) to replace it now than later when you find out there's a big problem.