Anyone know what this is?

Looks like a guage - maybe for the drive axle weight.

 

Looks like a pressure guage for the rear air bags that is fubar’d, chase the line back (and buy a new guage)

 

You have a PTO?

 

It's probably water and oil. I had a gauge the little rubber gasket on top and it came off. It's looks kind of like that.

It was a pressure gauge for a pump for doing tankers. Or maybe the trailer suspension gauge can't remember.

 

Some gauges are liquid filled for better accuracy. May still work and be a curate but Looks like the sunlight just baked the liquid and dial.

 

It looks like it is an unsealed gauge, normally if the gauge has a liquid, it is for inside use or heated space use, but that one looks like it has water or oil or a mix of the two in it and it is rusting inside (most likely water).

What is normally used is glycerine, and in a gauge it is used to dampen the gauge from mini-shocks.