20-inch Universal Slide Rule
Frederick Post model 1454 [1936-40]
Hughes Owens model 1772
Hemmi model 50-20
Top edge: 0-20 inches (Slant) 
Bottom edge: 0-50 cm (Vertical)
Back of slide: [S, L, T]
Back of rule: two copies of same table on plastic 

This page is from the 1936 (18th Edition) Post Catalog 
The term "Universal" is new and seems to be Post's term for its best products.