Magneplan Unitrac 1

This is a unipivot design with a low mass aluminum arm tube. This is the only arm I know of where part of the counterweight can be in front of the pivot point. The pivot is actually in the large semi-spherical counterweight. From the picture it can be seen that the arm post is some distance in front of the pivot point. This means that if you are thinking of mounting this arm in the hole cut for another arm, you can forget it.  No other arm I know of mount this close to the spindle.