Childcraft Nesting Chair Blocks, circa 1974
"They look like chairs, tables or steps, but quickly and easily build large structures. Each block is an open 12" cube made of 5/8" birch plywood, braced with a center panel on which a child may sit or stand. Hand cribs facilitate handling by very young children. Playboards enhance the building possibilities. Designed for indoor use." $9.50 ea,
Compatible Playboards of natural finish maple, 36" x 5 1/2" x 3/8" were $4.50 ea.