Please help identify this vintage (but non-Lego) brick!
That is a Kawada Dia Block made in Japan. Entex and Sears imported those into the US in the 70’s and 80’s and repackaged them under such brands as Loc Bloc and Brix Blox. The “S” is actually a K on top of a D, which is the emblem for the Japanese toy company Kawada. Dia Blocks are still made and available in Japan today.