This might be helpful.
http://info.nuwavemarine.com/mercrui...-buyers-guide/
"Which Riser gaskets do I need?
MerCruiser offers 3 different types of riser gasket. Full Flow, Restricted and Block Off
Wet joint
27-863726 Gasket – 4 Slot. “Full Flow” Commonly used with 3 and 6 inch riser spacer blocks.
27-863724 Gasket – “Restrictor” (with 2 small holes and 2 slots). The 2 small holes always go fore and aft. These are the most common for raw cooled engines.
27-863725 Gasket – “Block-off” (without holes or slots). This gasket stops coolant flow.” Common for closed cooling systems.
Dry joint
27-864547a02 – Full Flow – Both holes open. – Commonly used with riser spacer blocks.
27-864850a02 – Restricted – One hole open , one hole closed. – These are the most common for raw cooled engines.
27-864549a02 – Closed – Both holes blocked. – Common for closed cooling systems.
The only safe way to know which on you need is to take your riser off. Parts lookup programs are often incorrect!"