I like the heat gun idea. Possibly screw the plate to a piece of wood first to make it easier to handle when hot.
Any similar plate of that era will be black printed with similar ink so experimenting with a scrap plate from something else would tell you how much you are risking the black by using a particular process. I certainly agree that it would be very easy to scratch the plate with any kind of metal tool but possibly a plastic tool (PVC, not acrylic) would outlast your finger nail.
How about a hot water pressure washer ?
David
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