This 230 mm counterweight shaft is designed to attach to the bottom of the ZWO AM5 mount and others that would accept a screw with M12x1.75 mm threads.  The shaft is 20 mm in diameter and is made of 304 stainless steel.  Use a counterweight (sold separately) that is made to slide onto a 20 mm diameter counterweight shaft (like this one).

A counterweight (sold separately) and counterweight bar is needed when the load on the AM5 mount is over 13 kg, or 28.7 pounds.  To attach this counterweight bar to the AM5 mount, unscrew the plug from the screw hole and simply thread the counterweight bar into the screw hole in its place.