Here's a picture of the nut:

http://images.oreillyauto.com/parts/img/large/ede/276016.jpg

It's for a 3/16" diameter tube- - - -hard to find these days!

The copper tube through the water jacket that goes up to the oil connector on the spray-oilers uses one of these nuts to crimp down on the tube and prevent coolant leakage into the pushrod chamber. There's a mating female adapter that works with the male thread to provide the crimping action as the nut is tightened.

I believe this is the female thread adapter that works with it:
http://images.oreillyauto.com/parts/img/large/ede/orly_265301.jpg

The early-style oil connector with the line that threads into it has a similar female thread to provide the crimp the first time the fitting is tightened onto the oil line.
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
Love your enemies and drive 'em nuts!