O.K., Beacon Hill's old and historic and enviable in its charm, we know that. But this particular townhouse at 9 Chestnut Street is particularly old and historic and charming. It was built in 1824, and was the home for several years of Civil War vet, Harvard professor and later long-serving Supreme Court Justice Oliver Wendell Holmes Jr. (we're sure Oliver Sr., the poet, dropped by as well). The younger Holmes moved out in 1889, and a lot of things happened until now, and the 4-BR, 3.5-BA, 4,915-square-foot townhouse finds itself for sale as of last week. It's asking $3,500,000.