It's an old-style trailer brake control. The newer ones are inertial, and don't have the hydraulic line attached. It's for controlling electric brakes on a travel trailer, etc. If you try to use the control unit with the hydraulic line attached on an anti-lock brake system, it will disable the anti-lock feature. The hand knob is used to apply the trailer brakes only, without using the tow vehicle brakes, a good feature for dampening out a sway, etc.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
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