my '76 5.2 had a straight rod as the the rotator arm that I removed and put the regular arm that you can buy from Layline or Murray's for about $20. My NACRA F18 has it mounted on top of the boom. Some have them on the bottom of the mast, really doesnt matter, as long as it works for you and doesnt bind when rotating.
BTW, save the straight arm, as it made a really good striker for a windsurfer tapered alum mast that I turned into a spin pole.