Bear in mind that the older ambi levers for the 9/.40 slides were machined differently to accept the older ambi lever plunger. The newer levers are machined to accept the newer plungers. (The springs remain the same.)
The older ambi lever plungers looked very similar to the "body" plunger, having a machined shoulder and semi-flat head (versus the fully rounded head of the body plunger).
The newer plungers are straight-walled (cylindrical to the top, with no machined shoulder below the head), which gives them a wider, flat head ... and the ambi levers are machined to fit.
Mixing the ambi lever & plunger revisions won't result in the intended fit.
Also, unless they've changed it in the last several years, the 9/.40 slides with the spring-loaded decock-only assemblies use the .45 ambi lever, which has an added bit of steel "pad" which serves as a "spacer" against the outside of the slide, to help prevent lateral (leftward) movement of the decocker assembly's body.
It's just a guess ... but if someone were to have used an old style ambi lever on a new 4006TSW (because they somehow lost the new one, and just got hold of an old one from an earlier 4006, for example), the new ambi lever plunger wouldn't properly fit inside the recess on the inside of the old lever, and the lever probably wouldn't be securely held (probably being subject to be pushed out from the end of the body assembly).