Hi, these were fitted to the BSA Martini .22 match rifles, It should have a knurled screw to clamp it onto a dovetail, but yours looks to have been drilled to suit the mounting in the photos, so will have lost some of it's value to the normal buyers. They can make £80 with a good eyepiece in original condition, The 6 hole eyepiece on yours is worth maybe £20-30 depending on the day.
ATB, Ed