1960 440IC - What is this thumb cock for?

[dmlinton]

The 440IC I picked up the other day has a thumb cock just forward of the engine oil filler/dipstick on the right side of the engine. Cranking the engine with this thumb cock open produces a flow of some black sort of oily watery (not creamy) stuff. Engine oil looks good. Initially, it was coolant coming from the thumbcock. My guess is that someone has put some oily preservaton material in the coolant system but want to make sure nothing to0 abnormal or wrong is going on.

Can anyone tell me what that thumbcock is for - is it for draining coolant from engine block?