Habit of not throwing away things have proven useful once more. This time its the worn Igus bushing from the Horst Links of the other bike.
Apparently the mounting hardware that came with the Monarch wasn't all that sufficient
Thin enough washers with >12.75-12.80 ID aren't exactly everyday items at the local hardware store either.
Rummaging thru the "discarded items" bin... ID fits like a T
...and a little pen knife work + sanding down later...