I disagree; I think it is very clearly the provincial crab mark introduced in 1838. (Evidence of bigorne marks on the underside of it would confirm this.)
I think there is no real mystery here. When new marks were introduced in 1809, 1819 and 1838, there was a grace period each time when old items with legal marks could be struck with a special census or inventory mark without having to be subject to assay or duty. As your plate does not bear any of these marks, it was evidently presented for sale sometime after 1838 and, lacking these marks, it was considered unmarked and therefore subject to assay and re-marked with the crab mark. I suspect that the crab, rather than the Minerva, mark was used because while the latter guaranteed the fineness of .950 or .800 within a very narrow tolerance, the crab mark guaranteed only a minimum fineness of .800; perhaps the piece fell just outside of the first standard tolerance, or the assayer simply trusted the old marks and used the minimum fineness mark to cover any possible variance.