I scanned this ad for ACU-Card from the Spring 1990 issue (Edition 2) of Topps Magazine.
Frankly, dozens of third-party authenticators have come and gone. You can see a great list of them here on the Collectors forums. What I’m more interested in is where that 1952 Topps Mantle card is today (by the way, there’s no way it’s real).
I’ve read that a coin dealer named Alan Hager started the company. Many collectors have shared that he wasn’t really a high-integrity character. But If you want to add a piece of hobby history to your collection, you can still find a bunch of ACU-Card slabs on eBay.