To locate the original factory-spec. mags, it required a bit of reading on three websites. My 10-rd. mag was opened last night:
"Zasobnik Uplny 83 Kal." from "Uhersky Brod, Czech Republic", $44 plus shipping via CZ-USA in Kansas City.
By the way, this mag has No crossed-swords, or any other markings I noticed.
Even when empty, it requires a bit of a very short fist thump to fully seat it, because it is brand-new. Today all fifty rds. of Tula 9x18 ammo worked perfectly.
The mag spring is so very tight (6 rds. max can be inserted by hand) that I'll buy my first "speed loader", if whichever type can fit onto these mags.