When replacing a manual safety assembly it's common to have to fit a new sear release lever in the frame so the decocking timing is within recommended spec for normal functioning.
If the ambi lever spring is mixed up with the manual safety body spring the gun will not function normally. Switching these springs typically results in the pistol decocking itself and remaining in the On-Safe position after each shot, rendering the gun essentially single shot. Switching the plungers will also cause problems, like really nasty & heavy decocking. Also, the older 3rd gen ambi lever plungers in 9/40 had a different ambi lever plunger head design than the newer ones ... (or the .45 plungers) ... and the ambi levers were machined differently to accept the appropriate plunger design. Care must be taken to match the appropriate plunger to the lever.
Here's a set of the current body plunger/spring and ambi lever plunger/spring. The ambi lever spring is on the right. The ambi lever spring is painted light blue to make it easy to identify, but if you look closely you can see the wire is thinner than that of the heavier body plunger spring on the left. It feels noticeably lighter in tension when pressed between the finger tips, too, compared to the body plunger spring (the unpainted one in the picture).
Also, the body plunger spring has a shouldered, round head. The round head makes for smooth movement as it rubs along the inside of the slide's recessed angle. The straight walled lever plunger lacks the shoulder of the body plunger .... except ...
The early 3rd gen guns had an ambi lever plunger which had a shoulder (like on the body plunger), and while the head may appear round at first glance, a more careful look will reveal the head had a flat spot on the end (to engage the inside machined recess of the lever). Since that era lever plunger looked somewhat similar to the body plunger you had to pay attention not to mix them. I had to correct an early 3rd gen gun where someone had mixed up the plungers (but not the springs, thankfully). The lever plunger's flat head in the body plunger made for some really nasty & heavy decocking, and it was beginning to wear a noticeable groove on the inside of the slide's recessed surface where the plunger head ran.
Here's a picture of the different plungers, with the older lever plunger (shouldered, with flat head) in the middle.
Fitting a new sear release lever isn't all that difficult, but it does require knowing how to disassemble the frame and file the foot of the lever, keeping it at the same angle as originally produced (just removing metal to bring the lever within spec for proper decocking in any particular gun). Naturally, this means that it's not really a task for someone unfamiliar with how to do it and lacking experience.
Armorers used to be taught to check for decocking timing by eyeball, by manually depressing the levers to gauge the timing when the hammer was dropped ... in relation to the lever's position to the red painted dot and by snapping the hammer back to see if it would remain decocked, or would cock again ... but nowadays they're taught to use a set of 3 numbered metal drill bits as Go/No-Go gauges (the non-cutting end is the "gauge"), positioning then under the left lever at a certain spot and then checking decocking function.
Checking for decocking timing and fitting a new sear release lever (if needed, and it usually is, especially in the older 3rd gen guns) ought to be a relatively simple task for a local gunsmith familiar with S&W 3rd gen pistols, or an armorer (if you know one) or S&W (or one of its warranty/repair stations).
