Joe,
I can't see the blade tips, but I'm guessing it's a cleaver style Merc wheel. These were used widely in the 70's and early 80's on TRS drives. If the prop blade comes to a point it's a cleaver. In the mid 80's the newer Bravo wheels replaced the Cleavers. They are much more efficient, fairly well eliminate cavitation getting on plane and turning and give an all around better performance. If you look on the inside hub of the prop (the area surrounding where the prop shaft would come thru) you'll see some serial numbers. Those will give an indication of pitch and diameter. If you can find those, you can then call Mercury Marine in Fond Du Lac WI (ask for Dennis Cavanaugh) and he'll tell you exactly what you have and what current replacement wheel in the Bravo line would be appropriate.
Good luck