Locking block peening on those spots on the underneath of the slides is not exactly uncommon with Glocks, but it's usually been observed to have occurred more frequently, and to a great degree, in the .40 models.
Glock has revised their locking blocks over the years, and we were told in one of my armorer classes that at least one of the revisions ought to have reduced the potential for such peening.
FWIW, one of my G26's actually developed more peening of this type than my G27.
It started rather early in the gun's service life (and has now seen more than 13K rounds fired through it).
I've seen a little develop in my newer production Gen3 G26, but then it's only seen a couple thousand rounds, so far.;