Recently got a new bike with SRAM brakes I didn’t like, so went to swap in my old Zee brakes. The rear rotor for them was 203mm, so got a 203mm adaptor (the rear brake mount is post mount, and not IS mount like old bike’s rear brake mountings).

I didn’t account for the fact that my new/current bike’s rear post mount is set up for a 180mm rotor with no adaptors, so the brake with 203mm adaptor doesn’t fit, leading to much sadness.

Any idea what adaptor I should have bought/should buy to remedy the situation and fit it properly? Would a 180mm post to post adaptor work, on the basis that would add the 20mm needed?

You need a +20mm and a couple of spacing washers to make the extra 1.5mm up, or the hope 160-183 adapter.

As above you need the Hope post to post 183mm adapter