What is/was a stereotype in hot metal?
If you want to make a printing plate from a forme of type, so you can
machine a very long run, you can work from a mould rather than putting
the original type through the press. Or if you want to make several
duplicates of a forme of type so that you can work off multiple
impressions of the same page from one pass through the press.
Put the forme in a shallow wooden box, pour over it a kind of
papier-mache mixture called flong. Leave to set. Peel off (it's
slightly flexible) and use to cast a replica of the type-surface in a
variant on typemetal. Trim to size and mount to typeheight. Has the
advantage that you can curve the mould to cast a curved printing plate
for a rotary press - this is how all newspapers used to be produced
until the 70s.