Hi Mike, Patrick, and all I hope you are well. It's been a while since I last posted, since which we've had a few shakes over here!
You can check this thread - amongst others - for discussions and pics on this previously:http://www.sscycle.com/tech/viewtopic.php?f=19&t=9719
Having owned a few of these frames, my estimates are as follows:
-52 head casting, no date code = 1952-early 1953
-52A head casting, has date code = late 1953-probably early 1955
-56 head casting (with the top stearing head bearing cup raised above the backbone as per later sportsters), has date code = I believe late 1955 (ie the start of the 1956 model run)
The earlier frames have the hollow foot peg mounts.