These are some of the non-LEGO pieces I had in my LEGO collection when I was a child, and I was told they were old LEGO, but any site on the history of LEGO completely failed to show anything like this, so they have to be knock-offs.
They are likely made in the 60's but it could be earlier. It's compatible with LEGO, except that the studs are higher, so you can't put a thin brick on top of them. The plastic is softer and just slightly translucent. I like their solution for the underside, very neat, and a clever attempt to pass by the LEGO design patents. I don't know the manufacturer, and would appreciate any information.