The bolt hold open actuator fits between the lower and upper receiver. It interfaces and allows full operation of the bolt catch. Without it, the green follower of your magazine locks back the bolt - until you remove the mag, then it closes thus requiring you to manually use the bolt catch when pulling back the bolt. When you get the evo mag, you should purchase the BHOA, it's only $20 over here and is designed to work with the evo mags. The only other mod you may need to do with the BHOA is cut a coil or two off the bolt catch spring, I did, and I gain complete functionality when I use my conversion bolt.
Originally Posted by adymorris
Webley Vulcan .22, Webley Tempest .177, Beeman Kodiak .25, Beeman R9 .177, Weihrauch HW30 .177, Slavia 618 .177, Colt Commander .45ACP, Browning Hi-Power 9mm, Bushmaster AR-15 5.56/.223