“Signature” oysters have become a hot menu item for restaurants looking to bolster their seafood cred. Chefs partner with oyster farmers, who devote a section of their waterways to harvesting bivalves that are more or less unique to the restaurants and carry the name of their choice. Over the past three months, two more “private-label” mollusks have come on the market: the Big Daddy from Passion Food Hospitality and the Salty Wolfe from Hank’s Oyster Bar. Here’s the rundown of the signature oysters around town.