Stencil

[Ken S]

Keuffel & Esser made the Leroy lettering sets back in the day. They had many specialty templates including Greek and Math. Not sure if you could use an engraver with one of their scribers though.

There were some specialty systems made with a scriber that had an engraver on it, but you are not likely to find one of those.

Since they are designed to hold technical pens you could probably create a point which would fit that could work to lightly etch the metal.