yoopereh - That's because it's made by Bushmaster to Remington specs.
R-15 = Bushmaster
R-25 = DPMS
They are all owned by the same company Cerebus.
jmak - The gun shop I ordered mine from has a riser for AR's for $40.00 I believe he said were made by Nostar (I'm not sure it's spelled right) he puts on his personal guns. They have round knobs for quick release with a slot in them for a screwdriver(or a quarter).He swears by them.
-Other people put high or extra high rings on.
-After tossing a bunch of ideas around I spent the extra money and bought a LaRue tactical one piece mount (SPR?). Mostly because it's the most expensive gun I've bought , and probably the last,(I have one of everything I need now) and I didn't want to screw around spending money on mounts I wouldn't be happy with and end up spending more money in the long run....The Larue quick release is an awsome mount and I'm glad I got it...
-The Model 1 one piece comes up in a lot of posts, I'm not sure who makes it, and it sounds less expensive...... DON'T forget to post pictures when you get it set up!!!