https://www.wescooutfitters.com/p-450-tip-off-base-to-9m-ruger-1022-1-piece-black-matte-by-weaver.aspx
It is called tip-off scope rail.
If you want mount the full length scope rail on it you need to cut the bottom a little in order to let it sit on top of the loading slide top-cover. I think Ken has a picture of his scope rail.
Another thing when you assemble the gun
DO NOT! over tighten those two screws holding the plastic magazine window!! A very little pressure will crack the plastic.
Have fun!
http://www.network54.com/Forum/113813/thread/1214271949/Info+Needed+for+XS-B5-10
http://www.network54.com/Forum/113813/thread/1212558651/B5-10+pictures
