Turns out, that is not a brass plug but the newer thermostat design.
Here is a nice video on removal:

I recently purchased a 1956 Continental with a 1959 Mercury 430. Haven't pulled the water pump yet to see if the block thermostats are in there. Given that old thermostats are generally unreliable, I plan to remove them if they are there. My question is on the best configuration. The video on Engine...