This is a 40-X (earlier model) and the correct hand stop is round, approximately 3/4" thick and about 1 1/2" in diameter. It's been a while since I looked at any of mine but there is a threaded bushing that is inserted through the slot and under the rail. Then the screw that goes through the hand stop screws into the bushing pinching the rail to hold it in position.
There is another factory hand stop that is contoured to better fit the hand. Both types pop up on Gunbroker and Ebay on occasion and both types have a swivel attachment for a sling.
Last edited by navlav8r; 02/20/14. Reason: More info