Thanks they were very helpful. I am still looking for some help in installing the release hook to convert my pull to a release. There appear to be lots of little parts and I do not want to screw it up.
To understand how the release conversion takes place, you need to understand how a 391 (or 390, 303, etc.) trigger works. On the trigger are two spring loaded hooks. One is on the front top part of the trigger (call it hook #1) and it faces the rear. The other is a separate hook in the back of the trigger which faces forward (really called the sear but we'll refer to it as hook#2).
If you look at the hammer, you will see notches on both sides. The notch closest to the bottom of the hammer is the one that engages the small hook on the front of the trigger (hook #1). Since the trigger is on a pivot, when the forward hook engages, the rear hook (hook #2) disengages, and vice- versa.
When the hammer is cocked, pulling the trigger causes hook #1 to disengage the notch on the hammer and allows the hammer to move forward and strike the firing pin. After the gun fires, the bolt moves rearward, and forces the hammer back to it’s fully cocked position. As the hammer travels backward it contacts hook #2, pushing it aside and then allowing it to snap back to re-engage the notch in the hammer and lock it back. Hook #2 is important because when firing successive shots, the bolt pushes the hammer back before the shooter has had time to release the trigger. Because the trigger is still pulled back, hook #1 has not moved back into position, and cannot catch the hammer. To the rescue comes hook #2, which has moved forward, and catches the notch on the other side of the hammer the same way hook #1 did. . Without hook #2, the gun would become fully automatic and fire successive shots with one pull of the trigger.
Now to the conversion part. If you replace hook #2, with a specially designed release hook, the trigger will work as a release trigger. The release hook is longer and set at a slightly different angle than hook #2, but otherwise is similar. When the trigger is pulled back the hammer begins to fall. but as it does the release hook has moved forward (just as hook #2 did) and catches the notch on the hammer-- stopping its travel and holding it in a cocked position. As longer as the trigger is held, the hammer will not drop. When the shooter wants to fire, he relaxes the pressure on the trigger, allowing the trigger return spring to move the trigger forward. That action disengages the release hook from the hammer, and allows it to continue moving forward striking the firing pin. When the gun fires and the bolt pushes the hammer back, hook #1 can now catch it because the trigger isn't pulled back,
That's how a release trigger in a 303,390 or 391 works.