How do I tell if my 1979 75 HP Johnson Stinger is a "large gearcase" or a "small gearcase" -to order a prop?

Thanks for the info on the correct prop pitch but the on line dealers ask whether this is a small or a large gearcase? What do I measure to see which is right?

small gearcase:
15 1/2" tall (standing straight up, resting on the skeg, measure to the top surface where it mates with the exhaust housing).
four water intake holes (each side)
gearcase diameter 3 1/2" (round part containing the propshaft)

large gearcase:
17 7/16" tall
three water intake holes
gearcase diameter 4 1/16"