Encyclopedia of German Tanks of World War Two (revised edition) by Chamberlain, Doyle and Jentz.
“The hull design remained unchanged, but a new ball-mount was designed to replace the letter-box flap MG-port. These ball-mounts were fitted to a proportion of Ausf A from August 1943, and were fitted to all Ausf A from late 1943”
The ball-mount was thus present on some of the Ausf A from the very beginning and on all of them later. The book does not say anything about the percentage of that “proportion” but that is not relevant here and now.