Update: I took off the installed rear adapter, it says on it "185 F ->" with the arrow and F presumably indicating the orientation. There are no other branding or marking on it at all.
I put on a Hope mount F (Post to IS 160mm) for the interim (a Shimano is en route), pad to rotor overlap is perfect as far as I can see.
Oddly the removed mount arms are only a few mm longer than the Hope mount arms, that matches up with the pad wear pattern too ... but if the mount is intended for a 185mm rotor I'd expect to see a 12.5mm difference with the 160mm rotor mount.
Presumably the difference must be made up in calliper designs, and the fact this adapter almost works with XT callipers on a 160mm rotor explains why the previous owner thought it was an acceptable setup.