I doubt that they should be subjected to polishing. Pewter has a tendeny to flake. Also consider the length of time that these artefacts have lain in the ground.
I would definitely clean them by removing the surface soil/dirt/crud but being careful to preserve the patina. This has a twofold benefit - it is proof of age, as well as being a protective coating to the underlying metal (a preservative so to say, which consolidates the item). I have stripped patina from degraded buttons and coins and then polished them. The results showed up a rough pitted surface which looked better when untouched.
Just my opinion
GTB
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