The term screw can refer to a prison guard. An example of this is seen in the folk song The Catalpa:
So come all you screw warders and jailers Remember Perth regatta day Take care of the rest of your Fenians Or the Yankees will steal them away
My question is, where did this term come from?
I have found this page which provides an explanation, but there are no citations as to where it got it's information from. There is also this page which provides two different explanations, both of which have sources.
Which is correct and are there sources to back it up?