I changed the slide stop on a 3913TSW after I noticed that the original slide stop was so close to the frame it was actually digging into the frame slightly in normal use.
Way back when ...
We were told that there ought to be a relatively even gap (front to back) between the inside of the lever body and the outside of the frame, approx the thickness of a standard business card folded over once. Roughly, meaning a rough rule-of-thumb. A swag, in other words.
We were also told that since recoil forces tended to propagate in all directions, that heavier recoiling loads might cause forces to travel through the gun and affect some assemblies ... like the slide stop lever. It was suggested that as armorers it might be wise to periodically check to see if the gap between the lever and the frame widened or narrowed, especially if higher pressure loads were issued/used. It was said a lot of use of 9mm +P and +P+ might eventually result in the deformation (bending) of the slide stop lever (relative to the lever assembly's pin). The bending might inward or outward.
We were told that if the gap ended up too wide, the plunger at the rear of the lever might eventually slip off the outer edge of the sideplate, which would introduce a functioning problem. The slidestop might not catch on the follower, either.
If the gap became too narrow, the lever might start excessively rubbing against the outside of the frame. That could cause the lever's inside tab to protrude too far into the magazine body (touch bullet noses?), and/or interfere with the timing/speed of the lever being able to rise when lifted by the follower (empty mag slide lock).
Now, back in those days armorers were taught how to put the lever assembly into a vise and position it so that the use a lead babbitt to judiciously whack the assembly would restore the original 90-degree angle between the lever and its pin.
A couple of classes later, the "adjustment" of the lever assembly was no longer being taught, and armorers were told to simply replace the whole lever assembly with a new one if the angle ended up out of spec on the original one. I'm guessing that someone figured out that some basic LE armorers (who are seldom also trained as gunsmiths) might get carried away and cause more damage than they might "fix" by whacking away with a babbitt. Easier to replace the assembly with a new one.
The same was said about no longer replacing the lever's plunger and spring, and just replace the whole assembly if something happened to a pin or its plunger. That was fine when we had plenty of lever assemblies to spare, when the 3rd gen's were in production.
Someone later told me (outside of an armorer class, just on the phone) that in early 3rd gen production the folks assembling the guns had what was called a "lollipop" gauge, which they could use to check the gap between the lever and frame, but I never heard of it being mentioned as something available (or needed) by armorers. Never saw one or saw it listed among the gauges available for sale in armorer kits, and the guy telling me about it sounded like he was remembering it from a long time ago. (Back in the days of the transition from the 2nd to 3rd gen's, in the AIP, or Auto Improvement Program. Dunno, though.)
I've whacked a very few slide stop assemblies in earlier years, to bring them back into spec, but as I recall they were in high mileage guns. Easier nowadays to just grab a couple of assemblies and try them in some particular gun, checking to see how they drop and fit into that gun.
Another spot to check for fit is the notch machined into the center of the lever assembly pins, to make sure it's not heavily burred along an edge at the far end (machining marks are common in the rest of the notch), or at the end of the pin, itself (which depresses the plunger when installing the slide stop lever in the frame). A nasty burr on the notch edge or end might peen a steel guide rod plunger or gouge a plastic (nylon) rod plunger. (No, I don't "polish" the notches or remove metal, but just deburr, if necessary.)
When in doubt, take the gun and new lever assembly to a gunsmith familiar with the 3rd gen's and let him examine and check things. The parts are getting harder to find (so you don't want to needlessly ruin one, let alone cause a functioning/safety problem with the gun, itself).
Oh yeah, about the older/thinner levers ...
Some folks liked the thinner slide stop levers and manual safety levers. However, the company didn't make them thicker, and with another serration of two, just to spend more money on the cost of steel and machining. Some shooters found the thinner lever surfaces harder to find and manipulate under stress.
