Pachmayr Decelerator Slip-on recoil reducer, size small, colour black.
Part # 04414
To fit stock dimensions 1-1/2" to 1-5/8" wide x 4-3/4" to 4-7/8" long
Boxed and as new with no marks.
I tried it on a lightweight stalking rifle, and it reduced the kick massively, but sold the rifle weeks later.
£15 inc tracked postage.