On OMC outboards, up to 1933 the first three numbers of the serial number are the model number. From 1934 through the 1940s, it was the first four numbers. The rotary valve twins ran at a higher RPM and developed around 5 horse power. The 1934 model 4022 Lighten developed 5.1 horse power at 3750 RPM. x The check valve model developed 4 horses at 3250 RPM.
