Velo solo do 6 bolt sprockets that fit on disc hubs so you could has an SS kit on the FH and a fixed sprocket on the disc side of a normal 135 QR cassette hub…
I considered that, but if I get along with it then the plan was to run one of these on the freehub threads
https://www.sjscycles.co.uk/componentstandem-specific-parts/dt-swiss-arai-to-disc-brake-adapter-for-140-mm-oln-98-td/ which apparently is the same dinensions/thread as a freewheel.
Old mtb
Ss frameset with horizontal dropouts or eccentric bb?
Chaintension might be an issue otherwise
Sliding (well, swinging) dropouts on my el-mariachi.
Just buy a 120 tracker hub and the spacers.
chainline (or brake with the arai adapter) would be out by 7.5mm each side.
Surly prices though, that’s champagne, my budget’s more White Lightning on a park bench. I could buy a new rim for less than the cost difference to a NOS DMR.