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I fear this is one of those situations where it is going to be hard to arrive at a satisfactory translation solution without knowing a great deal more about what this actually consists of.
As so often, EN refers to its function — it acts as a 'pad', whereas FR is very likely to refer to the form it takes — as you say, it might be a 'plaquette' or a 'plaque'; though given how incredibly thin it is, neither of those seems really likely. It might help if you have any clue as to what it might be made of? I could imagine a thin metal 'shim'...