It is precision potentiometer with very low resistance and very high ratio accuracy. It could be used for say in bridge with "zero reading" galvanometer to match 2 currents or voltages. It was possibly marketed for chemical labs doing thermocouples measurements.
I'd estimate it is nearly as good as low-end Kelvin-Varley divider. And can still be very useful and very stable thermally.
One clue comes from googling the "Rubicon Company Philadelhia". There are similar products (from 1960s) like "standard 1 Ohm", potentiometers, galvanometers (with mirror for light beam!) and other high end lab devices of the era.
If by charges you mean electric charges, then no, a capacitor does not store charges. This is a common misconception, maybe due to the multiple meanings of the word charge. When some charge goes in one terminal of a capacitor, an equal amount of charge leaves the other. So, the total charge in the capacitor is constant.
What capacitors store is energy. Specifically, they store it in an electric field. All the electrons are attracted to all the protons. At equilibrium, there are equal numbers of protons and electrons on each plate of the capacitor, and there is no stored energy, and no voltage across the capacitor.
But, if you connect the capacitor to something like a battery, then some of the electrons will be pulled away from one plate, and an equal number of electrons will be pushed on to the other plate. Now one plate has a net negative charge, and the other has a net positive charge. This results in a difference in electrical potential between the plates, and an increasingly strong electric field as more charges are separated.
The electric field exerts a force on the charges which attempts to return the capacitor back to equilibrium, with balanced charges on each plate. As long as the capacitor remains connected to the battery, this force is balanced by force of the battery, and the imbalance remains.
If the battery is removed, and we leave the circuit open, the charges can't move, so the charge imbalance remains. The field is still applying a force to the charges, but they can't move, like a ball at the top of a hill, or a spring held under tension. The energy stored in the capacitor remains.
If the capacitor terminals are connected with a resistor, then the charges can move, so there is a current. The energy that was stored in the capacitor is converted to heat in the resistor, the voltage decreases, the charges become less imbalanced, and the field weakens.
Further reading: CAPACITOR COMPLAINTS (1996 William J. Beaty)
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Apparently, they are obsolete "Metallized polycarbonate capacitors". Not sure how critical is your application, so you may or may not want to trust an ebay listing.
I also found this datasheet for MKB3 and MKB5 capacitors. May be useful, may be not.