Have you dismantled it to see what the innards look like? Surely there's nothing to lose as you have no idea what it actually does.

Difficult to see how it could be a pump or engine without a meaningful input / output shaft. Looking closely, there actually appear to be 3 connections including a low pressure hose barb. So possibly some form of steam injector?

I have lots of ideas, probably all wrong, I'm sure it depends on whether or not the two sides of the imagined piston are connected and how.

I wonder if two gasses or liquids are connected to the two big connectors, and a pressure gauge or outlet tube to the push-on connector.

Looks like something designed to test small volume pressure and vacuum gauges.

Martin

Sounds very feasible. However, I'd have had it in bits hours ago just to be sure!

Pressure test pump for gauges you place a gauge that is calibrated and checked by your boiler insurance company inspector that has been sealed by them you place your gauge in the other outlet as you wind it in the pressure should be the same if not it allows you to remove glass and adjust the tube travel many years ago I used to use one about 5 times per week it uses hydrolic oil

Further to my post on this subject two points should be noted to fill with oil you screw the valve closed as you slowly add oil to the port furthest away from the handle you open fully by which time the barrel should be full . As for testing Vacum gauges ours was equipped with two fittings that went between the gauge and pump this was to set the master gauge and stop oil getting in the pump fittings were supplied by National Vulcan Commercial insurance company