1/4" plug in the cylinder head

[244ENBN]

I'm in the process of rebuilding my M-37s engine and noticed today there is a small hole going all the way through the head with a 1/4" plug on top of the head above #6 cylinder. My buddies 58' civilian has the same thing. Part of the casting Process maybe ?

[MSeriesRebuild]

It is there as access to determine when the piston is at top dead center on the compression stroke as a reference for timing the engine.