OK I'm trying to figure out what the 4 holes are for in the 500 EFI Mercruiser Thermostat gasket that goes towards the intake side.
Anybody know?

Nails !

Just kidding.

Those small 'rivets'ground the t-stat housing to the intake manifold so that your coolant temp sensors ill work.

So, don't put rtv on them which will keep it from grounding.

Thanks SB that makes sense - On my 500EFI, the Temp sensors for the gauge and the computer are located in the intake manifold.

Yup, most mercs have one or two in the t-stat housing.

crusader uses one brass button hole for a T stat gasket

Mercruiser also uses this same gasket in the pic from the OP post number 1 for carb apps as well --- anyone want to take a guess for a little trivia as to why would Merc use this same T stat gasket for carb apps to ?

my first thought is sensors can go in carb thermo housings too, but I have a feeling it is gonna be something goofy...

To protect the housing from electrolysis corrsion would be my guess