It is several decades old - got it second hand. Was in pretty rough shape. The manual's exploded view shows that, at some point, someone had reversed the position. Very hazardous - I was using it in the horizontal position, and a slight bump caused it to drop. It also interfered with raising and lowering the head, as the arm would have to be moved out of the way of the cross supports behind the carriage.
I'm simply going to use a smooth pin, with a little cap nut, to put it on or take it off - though would like to see if indeed the arm is supposed to 'snap' into a stowed position.
Thank you for looking