There's another way to tell with small stamps. If you have an SSA (small stamp A) it can be either 7/16 or 1/2. If you have an SSB it has to have a 1/2 hole. See Zenstat's Avedis Gallery. http://black.net.nz/avedis/avedis-gallery.html
Just to tie a bow on it and confirm what you’ve said, this one is definitely an SSA. I’m charmed by the amount of variety in the grooves and ripples in this one, as well as several “inclusion” impurities.