what is this plug?

[killerbrooks]

I want to add aftermarket gauges to my car and I'm trying to find a spot on the block for the temperature gauge. I am totally a novice when it comes to this stuff but does anyone know what this plug with the square is, and will using this spot work? I took out one of the bolts like the one next to the one with the square but none of the brass fittings would fit that hole. Thanks!

[imr]

hard for me to tell but maybe coolant. 

[MC1of80]

Yes, that will work. It may be difficult to remove. Make sure you drain the coolant otherwise you will have a mess on your hands.