The slide stop lever assembly is tensioned downward by its plunger being positioned and compressed against the small angled plate of the sideplate. The angle of that plate keeps the lever pushed down, and from rising up unless its "overcome" by the magazine spring lifting the follower upward (such as when the mag runs dry and the follower locks the slide back).
If an empty mag is removed from gun, meaning the follower is no longer pushing UP on the inside tab of the slide stop lever, the pressure of the lever's plunger (against the sideplate's angled plate) will gently push the lever down (because nothing is pushing UP against it anymore).
In order for the sideplate to perform its intended function of properly controlling the tension of the lever's plunger, and the normal downward movement of the slide stop lever, the sideplate needs to be properly fixed in position.
This is accomplished by a set of "prongs", or "legs", at the bottom of the sideplate being captured around the "headed" sear pin. (Meaning there's a recessed, machined groove around that end of the pin.)
If the prongs
aren't securely snapped down over the head of the pin ...
... or, one of the prongs have been bent, or even broken off, (don't have a pic of that condition handy), then the sideplate can't be securely fixed in its position, and it will be "loose" at the front (small angled plate), because it can rotate around its "pin" (which passes through the frame, securing the hammer, ejector and 2 other levers).
Typically it's the rear prong that gets tweaked, bent or snapped off. This may sometimes happen a grip is being improperly installed, where the top/left edge of the grip gets caught under the front/bottom edge of the sideplate, and the sideplate is forced upward. That improper leverage puts excessive force against the rear prong, and damage can occur. Once the prongs are spread, it's time for another sideplate. (Trying to "re-bend" the damaged prong usually results in it snapping off.)
Once the front angled plate of the sideplate is no longer firmly fixed in the correct position, but can shift upward, proper tension of the slide stop plunger can't be maintained, and the lever may lift higher than intended during recoil, locking the slide back early.
While it's not uncommon for the plunger head of any particular slide stop lever to reach almost to the bottom of the sideplate's plate, it ought not drop below the plate (where it becomes trapped below the plate). If
it can't rise, the
lever can't rise, as designed for normal operation. (Similarly, the lever assembly's lever ought not be bent or tweaked so the rear of the lever sticks out away from the frame too far, which could allow the lever's plunger to snap
outward off the angled plate, which would mean no more tension on the plunger.)
In some pistols, the lever's plunger might even come to the bottom-most position where
part of the plunger head may be slightly below the bottom edge of the angled plate. We were always told that as long as the center (and most) of the plunger head was still resting on the angled plate, that it was within spec. Interestingly enough, I've only seen that happen in a few of the .45's.
Normally, armorers are told to
lightly - (meaning
don't cause damage you're checking to see if it exists) - use gentle finger & thumb tip pressure on the top & bottom of the small angled plate of the sideplate, making sure it can't easily be wiggled up or down.
