AYER - How many ways are there to open an oyster? Several, if you use specialized knives produced by R. Murphy Knives, in business since 1850. You’ve got the Boston oyster knife, the Providence model, the New Haven, Chesapeake, Seattle, Gulf, and New York knives. Not to mention a new one, the Duxbury, developed by Murphy owner Mark Furman for Island Creek Oysters. “All oyster knives work,’’ says Furman. But shuckers have their preferences, and Furman, who, along with his wife, Mimi Younkins, bought R. Murphy three years ago, says, “You give the customer what they want.’’ If what the customer wants is a knife - almost any kind of knife - this is the place. Oysters-openers are one small category.