It's Busch Precision in Hartford sells all sort of CI surface and bench plate.This plate was last calibrated in 2016. When I contacted a service in 2018 to see what it would cost to have it recalibrated, I almost fell over. At the time they were asking in the $1,500.00 to $1,600.00. I can't imagine what it would cost today.
I was going to add a link to the Challenge website, but see they no longer make precision surface equipment. They now only make machinery for the paper industry.