Living in Italy, let me explain the situation round here in my neck of the woods.
Military calibers have not been permitted in semi-automatic rifles and pistols for quite some time, back from 1975 due to terrorism issues. However, the same calibers were permitted in revolvers and bolt action rifles... Yes, stupid, but it's the law.
Then in the following years, one caliber at a time, first the 7.62 Nato, and then the 5.56 Nato, then the 45 ACP were all permitted to civilians.
The only military caliber that is still a no-no here in Italy is the 9 mm Nato, or 9x19.
That is why we have the same semiautomatics in 9x21, firing the same bullets from the same magazines in nearly the same brass... again, stupid laws.
Re. other countries here in Europe, I believe that France has still the military calibers outlawed, so that receiver was probably due there.
Finally, EVERYTHING must be registered with the local police, not just the lower or upper receiver of a certain rifle. When I say everything, I mean handguns and rifles, magazines, any spare barrels, powder for reloading, etc etc etc... you got the idea!
P.S. Food for thought:
My grandfather had two shotguns registered with the police, as requested by law, back in 1943. When the Germans came to his town, they went to the local police, got hold of the firearms register, confiscated all the guns and rifles and made one big heap in the middle of the town square.
They then proceeded to drive over these guns with a tank, rendering them useless.