does anybody know what the benifit of this plate is on this #3 i have never seen one on a drive before............

That looks like a #5 drive.
Don't know why they put the plate on.

Mark

If the drive is run with high "X" dimension it tends to surface the props, the plate keeps down the rooster tail and keeps a little more water around the prop when running. I don't know anyone that's compared speed with it on or off, but since Merc is putting tem on the new XR's I guess it's worth something.

called a splash plate on my XR

I've never seen one of those added to a speedmaster.

My 3a's look like this......I think the picture you have is a IV or V drive.