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I have a small cast iron weight that is the right size with which make a
faceplate - is it likely to be reasonable stuff or more like window sash
weights?

Since there is no particular strength requirement for weights,
the metallurgy can vary all over the map, depending on what
other jobs the foundry has going on at the time. Lots of
places like to have extra "weight" molds on hand to use up
the remnants still in the crucible after major pours, so it
can vary from decent to the absolute grittiest, slaggiest
crap you have ever seen containing voids, rocks, etc.

If you don't mind risking an insert, chuck it up and make a
test cut and see how it behaves.