I tried a OEM Remington saddle mount on my 870. Trailing edge of the saddle on the ejection port side almost rubbed my hand raw when I shot slugs with it!! I brought it back to the store and told the guy if he could sell to do it. BTW, you can still see under the rail and use the bead sight if you want which I thought was pretty good. Anyhow, I'm back to the good ol' factory bead until I decide whether to tap the receiver and mount a rail or to put on a set of ghost ring sights.
http://www.remington.com/products/accessories/gun_parts/scope_mounts.asp
also, patience with remington's site, it is slow....................
edited to take out offer to sell saddle mount (spf)