You need the ambi lever plunger, not a body plunger.
Unfortunately, it also depends on the
vintage of the ambi lever in your gun.
The older 3rd gen's had an ambi lever plunger that looked like the round headed
body plunger, but which also had a small flat end (and a shoulder, resulting in a smaller diameter head).
The newer style ambi lever plunger is flat-headed and is cylindrical (no shoulder), with a wider flat end. The levers are machined to accept the correct lever plunger. The old levers won't accept the revised plungers with the wide flat heads.
Since your gun has the rounded trigger guard, I'd not be surprised if the lever is the newer style that accepts the cylindrical plunger with the wide flat end (part # 23775), but I'd still check.
This shows the standard body plunger (on the left) and the revised/new style ambi lever plunger (on the right). I don't have a pic of the
older style ambi lever plunger at hand, but it looked very much like the standard body plunger, with the difference being the tip was flat instead of rounded. Easy to mistake for the body plunger at a quick glance ... which could be a problem if the old ambi lever plunger ended up in the body plunger hole (as it dragged against the inside of the slide, making for rough decocking).
There's an ambi lever "kit" S&W lists that includes the ambi lever, revised plunger and its spring, but it's sometimes not in-stock.
If you call S&W customer service and talk to a rep, he/she might just send you a new lever plunger & spring at no charge. Might be able to tell by the serial number which vintage lever & plunger probably ought to have come in the gun (presuming nobody's changed it since then, of course). Worth a try.
FWIW, it's not exactly common for the plunger-type ambi lever to just "pop off" a gun, so I'd wonder whether the wrong lever plunger might have been in the gun, or whether the plunger and/or spring might not have been damaged/broken. The revision to the lever plunger (making it cylindrical, removing the shoulder) is said to have made the plunger stronger.