It could be the Benziger or the other issue of the same period. Benziger prints are on granite paper, the Girardets are not. You should be able to see little bits of colored thread (dark blue or red) scattered in the paper; it's easier to see them from the back. The Benziger 20c only comes perf 11 1/2 x 12. The watermark should actually be fairly clear as far as watermarks go.
Be aware that the stamp is reperforated at bottom. This is not uncommon with Swiss stamps of the period.
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