Mystery item from the estate sale of a long-term IBM employee. Does anybody know what this is? Is it an SMS computer card? Hand written numbers on the side include 247180K, 555261, 0198 (or 8610 if it’s upside down), and the letter H is handwritten and also stamped in a couple places. It is well used, with scuffs, scratches, etc. Size is 3 1/4 inches by 4 3/4 inches, and is 3/4 of an inch thick!