What is this thing?

I would say it's just a water separator. If it was and air oiler it would have a plug on top to add oil

 

Sure it ain't a sphygmomanometer ?

If it's not, it may surely raise the reading of one.

 

It's a water separator, Reitnouer puts them on all of their trailers, there's usually a sticker on the front telling you to drain it everyday too.

 

http://www.pneumatictips.com/2325/2...cs/what-is-a-filter-regulator-lubricator-frl/

 

After a little research both oiler and moisture sepperators look very similar. The easiest way to tell the difference would be to push down on the button and turn the base 1/4 turn. If there is a tube going down in to the bowl it is a lubricator if there is a vortex (looks like a small plastic fan blade that causes air to spin and throw moisture outward) or some type desicant filter then it is a water sepperator.