A bit of clarification:
1) It’s the right adaptor. I checked it side by side against a mate’s known good Shimano 160R last night.
2) The old brakes had the same issue, I just never noticed as they were second hand, so the wear pattern on the disk didn’t show. You also can’t see in through the back of those calipers. I pulled the pads out once to bleed the brakes, and they had a fair old step in them.
3) All the current braking kit is brand spanking – Rotors, Calipers, adaptors, bolts and pads. There are no steps in the pads etc.
Anyway, I’ve been out and had a mess with some of the old Magura mounts (160F/140R type, as my old Magura calipers seem to have +20mm built in), and some long M6 bolts. Put it together with 9mm (!) of washers like Avid used, and everything runs as it should, with the pad acting on the correct part of the disk. By that point, you’re adding leverage to the threads on the mount etc. No idea how much impact that makes, cos I’m not an engineer.