Well, it was that rare and cheap I felt like having an experiment- it couldn't get much worse!!
First thing- heated on a steel plate till 200˚C plus, when the carbonate changes to dark oxide, which is mechanically softer.
When it's up to that temperature and the colour changes, the coin is dropped into cold water.
The thermal contraction shock makes the black layer fall off.
Soft copper tools are then used which won't touch the bronze underneath at all.
I have copper brushes made from fine multistrand electrical cable, and stubborn spots can be flicked off
with a tiny copper chisel made from a piece of copper wire with the end sharpened to the right state.
If needed, contrast of the high spots on a flat looking specimen can be achieved by keeping the coin in your pocket for a bit!
And voila!!
I know it's all bit past what people like to have happen to any coin, but it was a satisfying experiment-
if it was the only F164A I had, it would be in the collection filling an expensive gap......:)