I picked this up in a trade this past weekend. Its not an item Surefire promotes very agressively, they made it so their M900 flashlight could be used on the Remington 870. The tripple rail fore end replaces your standard fore end, and retails at around $100.00. Can't say I would have purcahsed one for that amount, but the trade I did I got a good deal.
Ironically I mounted a Insight Technologies M6 flashlight with laser on it, instead of a surefire light. I also put a Falcon Industries vertical grip on the bottom to improve handling, I think I can cycle it faster now. I used the Ergo grip full profile rail pannel covers in OD green to cover the unused sections of rail. I put a green ergo grip and green telestock on the SST-870 so it would all match, and put a OD green SC4 cover on the OKO.
The end result looks like it belongs in a sci-fi movie: