The 5.1 has a rail attatchment that comes in the box, you need to punch out 2 holes in the bottom of the frame though (it comes with all the necessary stuff, so you don't need to purchase anything else) Although you'll find most people will buy a red dot mount that attaches by drilling and tapping the frame, rather than the undermount one like nineball makes. Plus I'm not sure, but I think the nineball rail mount only works with the 5.1 style rail, not the 4.3 frame.
This one is strictly for the 5.1. Plus it also rides much higher up on the gun compared to a frame mounted rail.
http://shop.ehobbyasia.com/nine-ball...5-1-black.html
This is nineball's other one, which is neat since it attaches via grip screw and slide release mechanism.
http://shop.ehobbyasia.com/nine-ball...pa-series.html