As Hawg said - many of the pre 1940 Sportsman pieces have NO prefix - however this only applies to those made between around 1935 and 1940. Prior to that the Double Action models had a 'D' prefix and the Single Action models had an 'S' prefix. My records indicate that serial number should be a 1st Model 2nd or 3rd Variation made between 1933 and 1936. It will be a 2nd Var. if firing pin is mounted in the frame (1933-34) or a 3rd if the firing pin is mounted on the hammer nose (1935-36). Both variations have a 1" long pivoting lever on the right side of the frame that unlatches the cylinder from the arbor.
Are you sure that the serial number (which will be found on the front grip strap) does not have a letter prefix?