Have you removed the seals and measured them? is their cross-sectional shape square?
is it these calipers?
There seems to be a lot of cross-over between brake parts, so the measurements can help. The other critical thing to know is which brake fluid is recommended in the master cylinder so you can match the rubber to the brake fluid type.
If that fails, there are a number of other rear calipers you can run. There have been a bunch of affordable WildKart calipers that have come up on ebay with mounting brackets.