BMCM, your pic gives a great example of how the cuts machined for the ejector depressor plunger channel and the firing pin channel intersected at small spot at the rear. A small bit of steel at that intersecting spot would often crack and break out, creating a "connection" at the intersecting spots. It's not considered a "problem".
In your pic, look inside the lefty side of the manual safety body cut of each slide.
In the carbon steel slide, the interesting cuts are still "separate" at the rear of the channel cuts, separated by a bit of steel.
In the stainless slide, the little bit of connecting steel has been broken out and left a space, connecting the rear parts of the intersecting cuts.
The first time I saw this in a 4513TSW slide, I thought the slide was damaged, so I called and discussed it (as an armorer) with one of the guys in pistol repair.
The tech sort of chuckled when I described the spot, and explained how it wasn't an unexpected spot for a small bit of non-critical steel to fail and fall out of the slide after being subjected to enough recoil forces. This was observed to happen in the earlier 3rd gen slides. (There's another spot inside the disconnector cut, on the bottom of the pickup rail, where a thin triangular shaped bit of steel will usually break and fall out of one side in older 3rd gen's, but new machining methods can now remove it during manufacturing.)
The tech told me that they had decided to start "pre-breaking" and removing that itty bitty bit of steel at the intersection of the ejector depressor plunger & firing pin channel cuts in the newer .45's, which probably explained why I'd seen it in a brand new 4513TSW I'd stripped to inspect. He further said they were planning to eventually introduce the same machining revision in the other caliber model lines, breaking out that bit of steel so it didn't have to do it later. Naturally, when I pulled some other slides from NIB .45's (4513/4566TSW's), to satisfy my curiosity, I found the new "open" spot where the cuts intersected.
Thanks for the pic. I saved it as an example to help illustrate this, if needed at some future time.