Look at the plunger in the pic above in my previous post and to the far right end of the plunger, sticking away from the camera is a pin about 1/8" in diameter and about 1/8" long. It just sets in the slot in the lockframe and there is nothing on the other side holding it in place. When the concave area on the right end of the plunger is engaged straddling the barrel extension rear tang dowel, that is what holds it in place. Look in the pic below to the left thru the bottom of the lockframe and you can see the plunger spring compressed and the right (or forward) end of the plunger sitting straddled the barrel extension tang.
Notice that there is a hole in the lockframe right beside the trigger the same size as the plunger. That's where the plunger slides back and forth while cycling so when you install the plunger you have to feed it thru that hole while seating the pin in the slot in the left rail. That hole is what retains the rear end of the plunger in place while the barrel extension tang retains the front end.
If your plunger does not have a pin sticking out the side of it, at the right (forward) end, then it is broken. I know this stuff can get confusing...hope that helps.