Bent birch ply. The cabinet is from Paimio Sanatorium, designed by Alvar and Aino Aalto in 1929. Widely regarded as the very first example of modern architecture applied to healthcare, some of the purpose-made interiors are some of the most iconic of the 20th century. Each bedroom had two of these cabinets screwed directly to the wall.
Illustrated in Alvar & Aino Aalto. Design, collection Bischofberger, p. 67.