Originally Posted by paulj84003

The majority of fail to lock open is a magazine problem, spring tension, follower. However, If somewhere along the line a mod 59 slide stop was inserted in a 39 you would have the same problem. The 39 slide stop should have a small arm that projects into the mag well about .270 while the 59 only projects about .150 so the follower will not contact the slide stop to move it up. The slide stop plunger might be stiff. Also the recoil spring guide plunger might be frozen or stiff. need to be lubricated.