What's the difference in these two props?

[thunderroad]

I've got a 69 Formula F190 19' w/160 Mercruiser sn 2497444. I use both 19 and 21 pitch props on occasion. I destroyed my 21 this weekend so I started looking around for one. Mine is part number 48-79576-21. I can find lots of them with part number 48-58424A1-21 which seem to fit the same model and look the same to the naked eye. What are the differences in these two?
Also...is there such a thing as a 20 pitch prop for this drive? The 19 gives me approx 4400 WOT and 32mph. The 21 gives me approx 3600 WOTand 37 mph. Is 3600 lugging the engine too much? 4400 seems to be screaming and the old girl IS almost 40 years old...although it had a new short block about 12 years ago. I'd really like to keep the mph around 34-35 if possible. It had a complete tuneup (cap,rotor,points,NGK plugs,wires,fuel filter,dwell and timing) about 20 hours ago and runs very smooth. Compression was an even 130-135 when checked in June of this year. Carb was professionally rebuilt last summer and works fine.
I could live with the 32 mph but hate to run it at 4400 to get there. The speed and tach figures are from the factory tach and speedo. No GPS.

 

[WillyBWright]

Re: What's the difference in these two props?

A lot of stainless props are available in even pitches. There is a 4-blade aluminum Alpha prop that comes in even pitches. Or you can have a 19 pitched up or a 21 pitched down to a 20 by a prop shop. Look just above your post and click on MerCruiser Props for a buncha possibilities.

I don't know the difference in the two props you mention except maybe one is cupped and one is not.