XS Sights Shotrail with a Picatinny Rail is for sale on Remington 870 Forum:

XS Sights Shotrail with a Picatinny Rail

XS Sights Shotrail with a Picatinny Rail

XS Sights Shotrail with a Picatinny Rail

XS Sights Shotrail with a Picatinny Rail