Vtg Stencil US No. 51
The Vtg Stencil series of fonts from astype are based on real-world stencils.
The US No. 51 design was drawn from authentic 1 inch interlocking stencil plates manufactured by General Stencil Inc., New York primarily for use by the U.S. Army. These stencil plates were in use from about the 1940s to the 1960s. However, there are also a few examples of similar stencil letters that existed around 1920.
In addition to the various stencil cutting machines, which have different designs too, the interlocking stencil kits are part of the NATO or National Stock Number (NSN) system. Since 1963 the number for the one inch stencil kit is 7520-00-298-7043.
So part number stayed but the manufactured stencil design changed. Other sizes have other stock numbers.
Enjoy the vintage!
If you are interested in the current stencil design the U.S. Army uses, please have a look at Vtg Stencil US No. 72.