Mike, the 17 stamp is the gauge of the bores. On breech loading shotguns the bores were measured 9" from the chamber, on muzzle loaders they were measured at the muzzle. Your shotgun doesn't have chokes. The rules of proof say that muzzle loaders with more than one barrel have to be proof fired twice, both a provisional and definitive proof firing. If the maker applied in writing at the time of submission they could be granted an exemption so it only received a definitive proof firing. Your Parker had that done. The VBP under a crown is the single proof stamp the Birmingham proof house used to show that. Otherwise it would have both a provisional and definitive proof mark.
There were several gunmakers named Parker in Birmingham and the Provincial trade that could have made yours. Take a close look at the top of the rib and see if there is an address and maker name there as well. From that we should be able to give more details and a time range the business was operating. Then again, maybe not. Sometimes the name was put on by makers for retailers.
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