Froggy -- US Import laws require all imports to be in a "sporter" configuration, one component of which is single stack mags. So AKs, VZ58s, etc, imported into the US come w/ single stack mags. Different than Europe.
CSA sends polymer 10 round single stack mags w/ these sporter rifles; many are converted, but all aren't...
Typically all machining is done for double stack, except that the sides of the magwell openings are more narrow and need milled out. I haven't inspected a VZ58 sporter in person however to confirm that's the case here.