VH,
Those old 67-1/2 volt "B" batteries were common back in the portable tube radio days. I remember seeing them made by Burgess and Eveready. They may still be available from someone like an industrial electronics supplier from a company like Eveready, but don't count on it, and if they are they will probably be $75.00 or more each! One good option would be to get a DC to DC converter module that would work off a 6 or 12 volt gel-cel storage battery, both small enough to fit the space. A faster option would be to get 8 regular 9 volt xistor radio batteries, like Duracells and solder a pack all in series. Hold together with tape. Put electrical tape over all the ends to cover the terminals. Mind you at the prices some places charge for a 9 volt Duracell you could still be paying $40 for this battery. How about trying this with some regular so called HD carbon zinc 9 volt batteries first to see if the instrument works. Sometimes you can find these for two for a dollar.
For a connector you could rip the terminal end caps off two 9 volt batteries, solder one to your battery pack and the other to what is in the instrument presently with short flexible wires. Be careful to note polarity through the connectors.
