Re: vz 52 rifle?
The tab at the rear top of the magazine activates the bolt hold-open when the magazine is empty. It should come through the hole in the back of the magazine and move freely up and down.
The reason the rifle won't feed may be interference from the bent tab or it may be that you are inserting the magazine wrong. You have to put the front in all the way first, then swing the rear end up and in, exactly like the M14.
I am pretty sure your rifle is in 7.62x45, but a chamber cast would confirm that. The 7.62x45 chamber won't "swallow" a 7.62x39 round; the shorter round wedges in and is flush with the end of the barrel, so it is not picked up by the extractor.
7.62x45 ammo is available, though not as cheap as the 7.62x39. Google "7.62x45" for some sources.