back to topic but maybe also Off topic but had to think about it:
What if they used the sheet to set up the machine for the real test prints?
The methods of production and relative amounts of production scrap to test print scrap are against it. And this is even conceding a test print of gray edges that no one can confirm.
When test prints are run you typically only want a few copies. As a result, test prints are normally done on pilot scale (prototype) equipment rather than the high speed workhorse machines. Test prints would also be run on much smaller pieces of clean stock rather than on large reused sheets. This is all a matter of economics. Pilot scale machines can be run by one individual, operate less than 50% of the time, and have minimal downtime costs. Production machines often require 3, 4 or 5 person crews, are normally overbooked, and have lost opportunity (downtime) costs of $100s per hour.
There were about 330,000 Unlimited sheets printed, and about 132,000 Antiquities sheets printed. Most printers at that time expected scrap levels of at least 1% to 2% at each printing stage. That means 4,600 sheets or more were scrapped while the card fronts were printed. Not each could be reused (scrap generated after a machine was running would already be printed on both sides). Even if each of these could be reused, that would still mean over 2,300 full sheets of generated scrap during the print runs. Since Carta Mundi hadn't yet ramped up its manufacturing commitment to Wizards, the Unlimited and Antiquities runs probably took a combined 6 to 8 weeks to complete. This means between 40 and 60 sheets were scrapped each day.
So you have thousands of full size sheets of production scrap, and possibly no full size sheets of test print scrap. It is much easier to go with what is known. I think you should be looking for another card set printed by Carta Mundi in the second half of 1993 that happpens to be the same size, and also has grey borders. It's printing was probably sandwiched between Unlimited and Antiquities. This would be a great story, something like the Charlie Brown cards from years later.