My old plastic coolant tank had those 2 holes. This new one doesn't. They were plugged until I replaced the tank a couple years ago.
My coolant sensor goes in the back of this tank. Its plastic.
Be real careful you don't cross thread the sensor. My threads were damaged and it kept leaking. I ended up replacing that sensor.
I just used teflon tape to seal everything up.
Picture posted sideways. Obviously the site glass is on the side, not the top.