Quote from The Pewter Socieety website, at
The Pewter Society - Pewterers? marks
"If you only have a couple of pieces, the Society is willing to identify the marks for you if we can. This service is free, but it is not available to dealers nor to individuals if the sole purpose of the identification is to facilitate the sale of the item."
Regarding DCMatt's guess that those might be fake silver hallmarks... you might have some luck here:
8 PDF - Hallmarks - fake and pseudo silver marks on pewter in 1636