That little oval plate is named by S&W, the Firing Pin Safety Lever Spring Retainer, or in the parts list, FIR PIN SAF LEV SPG RET.

Only the 9mm models DON'T have them.

All of the larger caliber models DO have them. (.40; .45; 10mm)

The 9mm models don't have them because the rear sight holds the springs and plungers in place and that's all that was needed.

When S&W designed the 2nd gen 45, the slide assembly was taller because the barrel was larger.

(The same is true for the .40 and 10mm models.)

The rear sight would still hold them in place, but now the rear sight was a little farther from them.

Now they faced a dilemma.

Make the springs and plungers taller?

Redesign the mag disconnect and firing pin safety mechanisms?

It was much easier to leave all the parts and safety mechanisms as they were and stamp out a little oval plate to make up for the extra height of the larger caliber slide.

No other reason than that.

So to repeat:

9mm models don't have them or need them.

ALL the larger caliber pistols (.45; .40; 10mm) have them because they have taller slides.

I hope this helps clear up the confusion and makes it easier to remember.

John