Current single sided (left side lever) manual safety assemblies can be installed in 2nd gen pistols which used the ambi assemblies, although it may also require that a new sear release lever is fitted and installed in the frame.
The reason is that the decocking function occurs when the top of the sear release lever indexes off the bottom of the manual safety assembly's cylinder as the safety lever is depressed and the assembly rotates in the slide, and lower leg of the sear release lever displaces the sear nose forward, allowing the hammer to fall. Changing the manual safety assembly may create enough of a tolerance change so that an existing sear release lever may not index off the manual safety assembly and 'time' the decocking of the hammer properly.
This can also happen when a sear release lever wears enough at the bottom of the leg and it can't push the sear nose away from the single action cocking notch on the right side of the hammer far enough to 'decock' the hammer. Time for a new sear release lever.
It's not a particularly difficult fitting, but it does require some knowledge and experience (as do most mechanical repair tasks
). Nowadays S&W trained armorers are taught to use a set of 3 numbered metal drill bits as Go/No-Go gauges (the smooth non-cutting end of the bit) to check for proper tolerance range when filing and fitting a new sear release lever. This is an easier thing to teach an armorer to do than by 'eyeballing' the decocking timing and checking hammer fall by slowly manipulating the decocking lever(s) by hand as used to be taught.
A gunsmith familiar with S&W pistols ought to be able to do it for you. The new sear release levers are made to much closer tolerances than they used to be, although it can still be a bit of tedious process to check the decocking timing each time after a couple of file strokes on the lever's foot. It requires complete reassembly of the pistol to properly check the fitting of the lever, and upon occasion just one too many file strokes can ruin a lever and require starting over with another one.
Sometimes an inattentive armorer can unintentionally change the angle of the lever's foot when filing on it, too, and adversely affect proper decocking.
Also, while most of the parts of the 2nd & 3rd gen S&W pistols are drop-in ... (with the exception of the sear release lever, extractor and occasionally a barrel) ... they still have to be checked for proper fit and function in any particular gun, and sometimes in order for a combination of parts to provide for normal function a given part may have to be replaced. It may not 'work' in one gun, but it may well work in another one (a different combination of parts). This isn't exactly something exclusive to S&W pistols, either.
Also, as you might imagine, the older production and manufacturing methods resulted in some rougher, less precise parts than are produced nowadays (to put it "charitably").
BTW, it is very important that the manual safety body spring & plunger are NOT mixed up with the ambi lever spring and plunger used in the 3rd gen pistols. The manual safety body plunger has a rounded head and a heavy spring. The heavy spring is needed to prevent the manual safety from rotating and 'decocking' during recoil. It does its job very well. The wrong plunger can create some excessive wear inside the recessed part of the slide where the round-headed plunger normally rides, and also make for some very rough, gritty and difficult movement of the safety/decocking lever.
Can you take the gun to a local gunsmith to check it out?
Just my thoughts.
