your mag hold open on the 742 is a button on the left side of the mag- if there isn't one there, it's the mag for the 760 series- I don't know about the new 750 series, but the 740/42/7400 mags will all fit- and if I remember correctly, the mag has to be empty for the hold open to work-it depends on putting the mag follower in the way of the bolt from coming forward- push the button or remove the mag, and the bolt rolls forward- it's more of a bolt release than a hold open