Something to consider when dealing with centerless ground stock is you can't determine the roundness with a set of mics. The process of centerless grinding can produce a shaft out of round way beyond the size tolerance and it will mic good. Every subsequent machining process will be affected by that . If the shaft mics in size and wont fit your coupler this would be an indication. You either have to deal with people and material you really trust or invest in tools to verify it yourself.