ShimanoTricolour br 6403 rear caliper is about 2mm short of the mavic open rim on my 86 raleigh ventua. Such a shame the tri colour looks really great bit im guessing there's no way of adapting it to fit.
there is a medium drop tricolour caliper as well as the short-drop race version. See if you can get some They do exist as I have some, and no, I'm not selling
(just to add, I'm talking about the single pivot version, not dual pivots
I have a pair of medium drop callipers kicking around in the tool box. Say £12 posted? One has a non-standard adjuster. They aren't Shimano but are extremely tidy looking dual pivots.