Ray that looks like a clutch head.. I know for sure that there are different sizes. You may have the wrong size driver. TMM will agree when he sees this...
Yes sir #20 is clutch head. There are at least 3 sizes. It has been years since I took a Holley metering block off. Looks like 5/32" as per post #3. Holley 4160 Rebuild - Special Tools Required?