On some boats, more than one transmission was offered from the factory, which will impact which prop should be run.
All transmission manufacturers will rivet a plate with the identifying information for the transmission to the external casing. An example of that plate is shown below. The portion we're interested in is the Ratio A = 1 B = 1, which indicates that this transmission has a 1:1 input to output ratio.
Although in most cases we can determine the correct gear ration without having you physically inspect your transmission, in some cases we may ask you to provide this information when contacting us to order a prop for your boat.