Help with fitting identification. Grease fittings maybe?

[GCamp0]

I have a number of these type of fittings with varying threaded bases. All of them have a protective cap. Most appear to be threaded 1/8" NPT. A few have unusual thread patterns, having the correct diameter for 1/4" NPT, but the thread pitch is the same as 1/8" NPT. I believe they are some type of grease fitting, but if so, I've never seen this type before. Any help is appreciated. 

[dustycrusty]

With a straight shot down the fitting and no check ball mechanism to keep grease or oil from flowing back through them, I think they are vents or breathers to allow air to escape due to heat expansion like the vents that were screwed into the top of driven axle tubes. Do those caps fit securely, yet loose enough to to allow wiggle room so air under slight pressure can escape out of the slots leading from the interior bore to the exterior, then down and out from under the cap?

[GCamp0]

Thank you for the reply. I never thought of that and it does make sense. The caps fit snugly, but not to the point where they would be air tight. They were found in a box with standard zerk fittings and Alemite pin-type grease fittings, so I just assumed they fell into that category. Trying to find a home for them. Best regards.