I have a set of large plastic toy bricks my grandmother gave me for Christmas about 1960. Red, 6x3x2 inches. Studs on top, matching indentations on bottom for good stacking. No manufacturer's markings at all. Original box did not survive more than a year or so, so I have no memory of any brand name on it. I think it was a white box with red print. Set was about 3-4 dozen & I still have about 20 or so. I have never seen any others of these; no other kids had them or knew anyone who did. Yet I am sure I did not get the only ones ever made. Anyone else ever seen them?

Copy/paste this number into eBay search:
171071236684
They are the only ones I see that might be 6" long, but they are still American Plastic Brick Co, I think, makers of the little ones that came in a cylinder.
Trouble is they aren't 3" high. But I have a feeling they are still that brand.