I have a NR CZ 858 with approx 3500=4000 rds through it. It shows EXACTLY the same four wearing surfaces that yours does.
Strip the receiver and slap the bolt without the carrier in place, and you'll see that the extractor contacts the upper side of the chamber mouth since it sits slightly proud of the bolt face. There are corresponding wear marks on the nose of the extractor.
Pull the bolt out and install the bolt carrier by itself in the receiver. You'll see that the two wear marks on the top surface of the receiver are caused by contact with the front corners of the bolt carrier as it slams into battery position. Corresponding wear marks on the bolt carrier.
Pull the bolt carrier out and place the bolt into position in the bolt carrier. You'll see that the single wear mark in the center of the bolt carrier lines up perfectly with the extractor when the bolt is almost in its rearmost position. The extractor flexes up and toward the bolt carrier during ejection, and I believe that this is the cause. The extractor is (should be) very very hard steel, while the bolt carrier is not.
EDIT: I was just reassembling my 858 and noted that the wear on my bolt carrier is caused by the rear corner of the extractor, not the front. My extractor sits with the rear corner proud of the bolt, and this surface shows wear corresponding to that central mark on the bolt carrier. END EDIT
I'm no expert, but just putting these observations together leads me to believe that all of this might simply be "within tolerance wear". This is a military-grade rifle not particular famous for its tight tolerances. Just looking at the locations, I don't expect that the wear that my rifle shows will be a functional problem. They are not lockup surfaces, and even so I'd expect the parts to "wear in" to one another to a certain point and then just stop progressing. The one and only concern I'd have with your particular rifle, OP, is that the bolt carrier might not be hardened appropriately (known problem with CSAs, IIRC). If so, the extractor wearing on the carrier could become an issue. The design has a pretty violent ejection cycle, as I'm sure you know. But maybe it will be just fine, since the location doesn't necessarily interfere with any function.
Some 858s and 58s probably have tolerances that stack differently and so might not show this wear. Some do. I'd guess that the rifle is designed to function within these tolerances. If someone has a report of a 58/858 that has failed due to this sort of wear, I'd be very interested to hear it. Nobody likes their baby to be an ugly duckling, but it's a military rifle designed for hard, hard use. Personally, I'm glad mine is scratched up and shows wear: Now I have no reason whatsoever to be concerned over its cosmetic state.
