While I agree that they are modern fakes, I do not agree with the present "lower" date (the '50s). They popped up in various places (mostly Mexico and France) in the closing decades of the XIX century and they were invariably associated with the flamboyant French self-proclaimed antiquarian Eugene Boban. More than a few experts raised the doubt back then: in fact while Boban tried to push his skulls as the product of a long lost civilization and was able to sell quite a few of them, he never managed to raise the huge sums some antiquities were fetching back then (for example an Egyptian mummy with its sarcophagus could sell for a small fortune). His skulls were considered nice, carefully manufactured curios but nothing more.
When the skulls came back to center of the spotlight during the "counterculture" of the '50s and '60s for some reason everybody forgot about Messieur Boban and his down-to-earth antiquarian critics. The skeptics were simply claiming that "only now we have the level of technology to do this, hence they are very recent fakes" while the believers claimed that only an incredibly advanced civilization could have manufatcured them. Both forgot that during the XIX there were already very advanced and powerful grinding and polishing machines available. Most jewelry shops in Paris, New York or London had both the means and the expertise to manufacture them.
But everybody had forgotten Boban and his travelling antiquarian shop.