I did some googling on the Internet, but did not come up with any
I took one of the black decals, and was able to
burnish the design onto a piece of paper.
Burnishing the letters (or symbols, etc) onto paper ( or vellum, or
other drafting film, etc.) is in fact their intended use. The stuff
kind of dates to before everyone could computer print anything they
wanted in any font they wished, but still have uses in the graphic
arts when clean lettering is desired somewhere where you can’t just
use your computer printer to generate the whole thing. So far as I
know, transfer lettering like this is still quite available, if not
quite as widely used as it once was.
The letters CAN be used as etch resists too. Some versions were
specifically made for this, especially for etching circuit boards.
But they don’t, as I recall, actually work all that well, tending to
lift off the metal more quickly than more standard resists do. As I
recall, the usual method for circuit boards was to use these things
to make a mask for exposing a standard photo resist, but people
could also use them directly with modestly good results on copper,
with ferric chloride mordant. Be sure the metal is very clean, of
course. And warming it a bit while applying the letters helps too.
Still, your results will vary…