According to the Standard Catalog of Smith & Wesson, 3rd, Ed., that should be the .32 Hand Ejector Third Model, manufactured from 1917 to 1942.
271,531 were made, with the SN range being 263001 to 536684. Your SN would indicate 1917 or 1918, most likely 1917.
That was a very nice find. I'm impressed!
Oh, FWIW, the book says the NIB value is $700.00, value for excellent condition is $385.00.