I did a search on Ebay and the corners are angle cut to prevent corner damage. I never heard of them. It would've been nice to have those when all cards were made of card board.
While I'd never seen or used them before, I generally use the extra wide sleeves that they make for thicker cards. They're a lot easier to use than the standard sleeves...
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ebay i.d. clydecoolidge - Lots of vintage stars and HOFers, raw, condition fully disclosed.