Mike;
I think you are simply not looking back far enough in history. To the best of my knowledge, although admitting the shot Cos do not consult with me, all shot produced today & in fact for a good many years does in fact have some antimony in it.
This I believe was not always the case & old catalogs etc for shot do indeed offer both Drop & Chilled & charts of number per ounce vary in their numbers, indicating a difference in the density (aloy or lack thereof)of the shot.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra