As Andy LaTorre mentioned, unmodified steel punches are going to simply
cut out a triangle for the A or a circle for the O and so on, unless
they are modified by filing out bridges across areas that, if left
unbridged, would let part of the stencil paper drop out. Even then,
would the punches be sharp-edged enough to cut through heavy paper
easily enough to permit their rapid and repeated use? I don't know.
Looking at any stenciled work on a piano word roll will suggest where
the bridging needs to be filed into your punch matrices, if you go that