Modulex was a spin-off company from LEGO. They made these bricks at a smaller scale, and different width/height ratio. You can compare them in this picture
The brick on the left is normal LEGO. My understanding is that the Modulex bricks were initially made for architects, to build models of their designs.
Later, they started producing signage - basically a huge baseplate mounted vertically then with letter printed tiles so you could make a sign (for instance, I used to work in a building where the building directory in the entryway was Modulex).
In the 60s(?) Modulex broke off as a separate company
, and now makes all kinds of sign products.