To me, they like as though they could the bottom fittings of more water gauges. The larger diameter threaded stub has a flat bottom recess which would take the bottom end of the gauge glass. The long threaded stub would thread into the boiler and have a locknut to secure in position once lined up. The tufnol? headed spindles could be shut off valves.
They seem to be lefthand and righthand fittings of a matched pair.
Could be totally wrong of course!
Have you got a photo of the other side of the fittings?
John