The Monotype composition caster uses various holders to position the matrix (which has the shape of the letter to cast cut into it) properly over the cavity of the mould. These are installed in the caster instead of a diecase of composition matrices.
One style of matrix, called a US (or “American” or “Lanston” after the original company name) display matrix is a rectangle of metal about 1⅛×¾×0.1″ (28×19×2.5mm) with two beveled corners. This particular mat holder is for holding such matrices when using an American/Lanston mould (the English moulds have the cavity positioned slightly differently and so have their own set of mat holders).
Each mat can only cast a single character so if you are casting, for instance, a font of type, you must remove the mat holder from the caster to change the matrix for each new character to cast.
A Lanston display mat holder insert
Actually removing the mat holder is a bit of a nuisance, as it requires stopping the caster at just the right place in its cycle, uses both hands, and is just generally fiddly to do. To make things easier, this particular mat holder has an insert which is easier to remove than the entire holder, allowing you to remove it one-handed, replace the matrix with your other hand, and re-install the insert all in a few seconds.
The trouble is that the mat holder I owned had no handle on it, all it had was a metal tang about 2 inches (5cm) long which, like the rest of the mat holder, got very hot after casting for a few minutes.
I had a few file/chisel handles which I think I purchase from Lee Valley Tools at one point—I think they were a short-run sale item at one time. They jammed onto the tang quite nicely and made a good handle, but it still occasionally came off, especially when I was actually removing the entire mat holder and not just the insert. You have to do this at first when “adjusting the quad” (i.e. adjusting the caster so the type comes out the correct width), and also when changing between casting type 19 points or less wide and casting wider stuff.
Just recently I finally got fed up with this and drilled a hole through the handle and tang, inserting a metal pin to keep the handle on. I haven’t used it yet, but that should remove one minor annoyance from font casting.