I’m pretty sure that the Cosmos holo Fossil Test Prints that I saw, had regular Pokemon backs and regular round corners.
More info on those at this link.
English Fossil Cosmos Test Prints
I don’t have any inside knowledge on that specific white back square corner card that you posted, so what follows is an educated guess.
Since CGC graded it, I’m pretty sure that the card is authentic, and the ink dots will match a normal Fossil Ditto card, because they’d use the existing Fossil printing plates for this kind of a Test Print.
Because of the white back, I expect this is not typical Pokemon cardstock. This would be whatever kind of cardstock this sample Cosmos holofoil was already attached to when the unprinted sample sheet arrived at the print facility. For the same reason, I suspect this is older than Cosmos holofoil Test Prints done with Pokemon card backs.
There are factory cut square corner cards, but I suspect the card you posted is NFC (Not Factory Cut).
If I’m reading this population report correctly, it looks like CGC has graded 107 cards from this sheet.
It was probably cut in Texas, and I’ve got a pretty good idea who would have cut it.