they look to be studs, in this pic it has no studs installed it seems, see the small holes.
some heat from a plumber soldering torch should help break the one that still has some sticking out, the other one you can try to drill a pilot hole and use a easy out but that never works out well, due to not having a flat area to drill into, thats into the stud.
bet bet may be to drill out the broke one, that snapped inside, and use a large bolt and nut.
i'd get a brand new drill bit for metal, maybe 2 of them and do your best to stay straight on the stud, and not stray into the metal around it. and some wd-40 to keep spraying on the bit and hole you are drilling. and stuff a rag in the inlet opening, you dont want anything to get in there.
