Interestingly (perhaps), the linotype moulds for each letter were always called "mats", short for "matrices". I never did work out why.

I can rarely be bothered to look up on-line dictionaries, so this comes off the top of my head. If I remember rightly, a matrix (besides its tabular mathematical use) primarily refers to a medium into which material (or shapes in this case) can be embedded. The contractions to mats is presumably simply jargon.