SE218 (Replaces Most Mercury 3.0L Counter Rotation) 

SE218 Counter Rotation
Replaces Most Mercury 3.0L Counter Rotation Lower Units
Do not use for standard rotation
13 Splines
25 and 30 inch Shafts.
If you have an 8 Spline drive shaft you will need to buy the stub shaft unit
and purchase an upper driveshaft from an OE dealer.
25" 45-823901A 1
30" 45-824115A 1 (This shaft is no longer available from the OE. You will need to either source a used one or repair your old unit.)