This is an original door & window box card insert from Hilary Page's Kiddicraft Self-Locking Building Bricks Set No. 1 from 1945-1947. It comes complete with 3 doors and 12 windows as pictured. 

Hilary Page's toy company Kiddicraft first patented the self-locking building brick in 1940. WW2 put the toy company on hold but immediately after the war in 1945, Hilary Page improved upon his original design by adding slots to his bricks for doors & windows. These bricks were made of Cellulose Acetate and were available in Red, White, Green, Yellow and Medium Blue. 

LEGO designed their Automatic Binding Bricks based on the Hilary Page Kiddicraft Self-Locking Building Bricks.

Hilary Page was passionate about child development and wrote many books on the subject. This is what motivated him to make toys out of plastic rather than wood because they were much safer and cleaner for children to play with.