Simple test to see if you want to change them is to look at the end bung. Glass seal and it will last essentially forever, a rubber seal it is time to look for a replacement, as it will have high ESR. Kemet still sells them as new, though you would in most cases be able to replace it with the same value and voltage 105C Nichicon or Panasonic capacitor with no ill effect. They were used because they were a lot more reliable and had lower ESR than the wound axial aluminium capacitors in the time the meter was designed. The quality of electrolytics has improved in the interim, leaving wet tantalum there only for certain niche uses where you need either the multi decade lifetime or the insanely low ESR along with ultra low leakage.
Cases are silver plated steel, or they use a tin outer plate with an inner silver plate over steel. The electrolyte is sulphuric acid. They are also made with high temperature solder, and the rubber bungs have the central wire that is showing before the steel lead made from pure tantalum metal, as it is the only thing that will not corrode in the acid bath.