Found an old Relco Frontiersman on ebay that was actually working,like I had back in 1969 so I decided to compare it to my Fisher F5.
Put a fresh 9 volt in the Relco set the threshold, and tested 3 coins. got my F5 and put the gain on 50% and ground balanced to the air. Tested the same three coins.

https://youtu.be/8onDZ9RCOnY