This was the Nikon S, long overdue when made available early in 1951; it is a Nikon M with flash sync contacts, two sockets at the upper left-hand edge of the body. All cameras sold with this feature are considered a Nikon S by the factory, even if marked M, though collectors do distinguish these as more valuable than the S. The Nikon S sold well, and became the first Nikon imported to the US market. A number of Nikon S cameras have one more serial number digit, known as the 8-digit Nikon S. When reaching 6099999 the engraver continued at 60910000, but it was soon realized that the long serial number was impractical, and after some 1200 cameras, the numbering reverted to 6100000. The 609 prefix in the serial number refers to the date the design was approved in September 1946.