I would expect the "c68" to mean Rc 68, but I wouldn't know myself. You're in Citrus County - right there in the Crystal River area. That is a nice part of the country - If you wish to "cheap out" on the test blocks, and can't find anyone around locally to help, send me an email and I can send a block with a value on it to give you an idea where your tester is at. Of course, best is to purchase a test block traced to NIST = but those are pretty expensive.
Ken H>