If I look at ther UK site and search by brand specific though I still get the G5 and then the two R/Rs
VN750 Vulcan 85-95 G5 RR26/RR20 REP1 with the notation at the end of the G5 specs..
"We also manufacture a hi-power version of this generator, they provide at least 30% more power than the standard generators and are essential where there is extra demand on the electrical system i.e. fitted heated handlebars, extra lighting, communications and GPS. Click here to view G105."
So the G105 is the higher output from what it says.
Regarding the R/R, I'd check to see if they are both 6 pin connectors. There is a Suzuki 5 pin R/R unit that also works on our bikes, but you have to switch the red and black wire to get it to work on our bikes (otherwise you blow the main fuse as soon as you fire up the bike)
If you look at the pictures of the two the RR26 has 3 yellow wires on top, the RR20 looks like it only has 2 yellows and a red which may be the wire for the bottom row.
I think the difference is in the wires and the connector. I'd go with the RR26 based on the visual.
Patriot Guard Rider
2000 VN750 Sere (Serendipity)
1990 GL1500 (Ole Blue)
1986 VN750 EVie (project bike, heavy custom)
VROC # 11628 / 25000-H
ARVROC # 12 Coordinator and Crowd Control
OKVROC # 18 (H)
TNVROC # 45 (H)