Second Street was a street that ran through Hill Valley, starting at the Hill Valley Railroad Station, intersecting with Main Street at Courthouse Square, and ending at the Town Theater. In 1985, U.S. Route 395 passed through Hill Valley on Second Street.
In 1885, the Second Street Gunsmith Shop was located there, as evidenced by a photo of Buford Tannen and the deceased Lazy J riders. Marty McFly walked past the gunsmith shop on his way from the railroad station to the Palace Saloon.
Although the stationery for Lou's Cafe showed an address of "Hill and Main", the Essex Theater was at that intersection in 1985, and Lou's was one block over at what would be "Second and Main", on the site of the old Palace Saloon. It is possible that Marty's letter to Doc was written on older letterhead from a former address.