I think the ST170 setup is unique to the RS and ST models, although there is a lot of commonality with the Fiesta ST rear setup as well.

Got any pics of the bit that you need? I'm slightly confused - the calipers itself are readily available and Ford sell the carrier - FINIS code should be 4346382 for the Focus, I know this should be readily available as people buy them when converting the Fiesta ST rear brakes to the larger ST170 setup.