This is a small decade capacitor made by HP. It is adjustable between 40 pF and 1.2 uF.
This is not just a capacitor substitution box , it also can be used as transfer standard. It is 0.25% accurate +/3 pF. The capacitance residu is 1 pF. To measure it correct it has a 3 terminal connection. I have tested it with some LCR meters and a vna to see how high it goes and it was still spot on and usable upto 40 MHz (much better as the specs, but, hey, it’s HP so no supprise).
There is a cabinet in the cabinet and that is connected to the outer cabinet for 2 wire or disconnected from the outer cabinet for use with 3 wire measurenments.
The real beauty is inside. All high quality mica and aircaps:
They look a bit grey on the picture but that is not how it is, you need sunglasses to look at them so nice and shiny. A mirror would get a minority complex from it. Down here a copy from the 1973 catalog. Mine comes from a cal lab and is bought there in the late 70’s and not used for over 30 years after the lab closed (but all gear stayed there)