You are looking at the normal casting marks at the bottom of the piston bores. I just did mine and they were exactly the same. Clean them up good and inspect the bore walls for damage. If they are smooth and the pistons move easily in and out, replace the O-ring seals and you should be good to go.Attached is a picture of one half of my GL650-#2 brake calipers. The front brakes were the most obvious thing wrong with the bike when I picked it up 2 weeks ago, and I'm wondering if this was a contributor. It appears to me, unencumbered by knowledge, that the back end of the cylinders have worn through to a mesh-like support area. Since I've never seen the inside of a Honda caliper, I'm not sure how normal this is. I don't know why there would be friction-type wearing there, but maybe after 70K miles that is what happens. All four cylinders have similar patterns. On a maybe-related note, one cylinder had the pile of goo seen in the other pic by the fluid holes in the back end. It has the consistency and color of moly grease.
The o-rings and pistons look OK, but the dust seals are just about kaput. Should I spend any more time cleaning and rebuilding these calipers?
(I got a "Can't use a .jpg image type" error when posting this. Has something changed? Took out the image code, so maybe someone can fix it.)