This is a specimen of non-ornamental rule and corresponding corner-pieces with matrices to be used in the Lanston Monotype Type-&-Rule Caster, the Lanston Monotype Material Making Machine / Junior Material Making Machine, or both. Undated, but probably 1922-1925.
For a discussion of the confusing way in which these machines are named in this document, see the original presentation of this digital version at:
http://www.circuitousroot.com/artifice/letters/press/stripcasters/monotype/sales/index.html
600dpi RGB JPEGs in a PDF.