There was no plate or milled out spot for a plate on my cs9
Seems like at the end of 3rd gen production the took that step out to save $
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The
spring plate underneath the rear sight base was originally used on the larger/taller .45 slides, although I did see it in one of the newer, late production guns in another caliber in some other 3rd gen, in recent years. It escapes me at the moment, but it was just one of those moments of "(Shrug) Hmm, so they decided to do it in other guns toward the end, huh?"
FWIW, I can't remember seeing them use a spring plate in any of the CS9's, or some other 9 s', even going back to the very earliest of the first carbon steel (slide) black CS9 models.
I don't know for sure (since I've never asked), but my initial thought would be to think that maybe adding the spring plate in the taller slides might somehow have been to add some extra material above the plunger springs to offset dimension differences in the taller slides? Dunno. Just a wild SWAG ...
Maybe the engineers finally decided they could add the extra cut during CNC production of other caliber slides without adversely affecting the tension of the 2 springs? Dunno.
I
do know, however, that having the springs covered by a spring plate is a really good benefit when you're installing a rear sight, as it captures the top of the springs and prevents the sort of damage that could often result in slides which lacked a spring plate, when the advancing left edge of the sight base being installed could cut, crimp, bend or twist the top spring coils.
Little wonder why they decided to keep the spring plate idea when designing the M&P slides. Except in the M&P's, the itty bitty spring plate used initially earned the nickname of "UFO", because of how easily it could slip out and go flying while trying to remove or install the rear sight. Some genius back there had a brain storm and figured out they could attach the plate and spring to make a single unit, so the plate wouldn't go flying if it slipped. Sheer genius (from an armorer's perspective, at any rate.

)
The wide oval shaped plates used in the 3rd gen slides were much more easily managed during sight removal and installation.