Thank you for your reply. I am actually very confident the shell is original. I have seen other examples of shell spoons from early to mid 19th century and I have a few other "modified" pieces as well - one is a really cool 1874 Russian handle with an 1847 Victorian Gothic Crown as the bowl (gothic crown most likely a reproduction but did test positive for silver). My biggest concern with the shell spoon is trying to find out if the pictorial mark can help identify the maker. What you pointed to as a solder line is a shadow on the curve from my poor lighting.
There may be a logic to an odd maker mark. We have several legal cases by manufacturers against retailers over accounts recievable not paid. A retailer may want to show a guarantee to a customer, while protecting his middleman position by not revealing the actual maker. But the maker still needs a way to prove his property should recovery become necessary in partial settlements (hard silver having some intrinsic value). A large-scale maker dealing with several retailers over a region might then select a special mark for this line of his business. Perhaps such a mark might be entered as evidence in a legal case, but not being revealed in public records of that case (trade secrets).