Hi everyone I have several pieces like this that have the square bottom “excuse the pun “ rather than the round bottom. It seems very old and to be honest I can’t remember from my childhood.
I've seen these before too (some come with a quarter-round corner) and are quite susceptible to discoloration, to the point where they can seem to be Tan!
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https://www.brickowl.com/catalog/lego-plate-4-x-8-with-waffle-underside