You'll find out the voltage when you get the bike. Even if there's no battery, the bulbs will be marked with their nominal voltage. Honda started converting to 12V/CDI in `82. However, the North American-spec CT70s weren't 12v until the `90s, that includes the rare `83-`90 bikes.
Note that your new old Dax is an AB23 model, not an ST50, yet it has "ST" markings. I believe that is the original seat, for example. It's one of those persistent monikers that sometimes causes confusion.
Also notice the GVW spec on the VIN tag...255kg(!)...that's 561lbs, or ~400lbs capacity for rider & passenger (the bike, itself tips the scales at ~160lbs), almost double the stated value for the CT70. Since they both have the same frame, something else has to account for the grossly different values. I very much doubt that the bone-stock suspension can handle that. My point is that the basic structure of the bike can, if the shock & spring rates are increased. It's a non-issue for anyone after a 100% original bike and a minor issue of sourcing heavier duty shocks and using thicker oil in the forks, for owners over 150lbs who want to ride a stock, or nearly-stock bike without bottoming-out the suspension. However, for those contemplating a roadworthy bike capable of 50mph, or better, and carrying an average adult weight rider, plus the occasional passenger, which would place anywhere from 200 to 300lbs+ on the bike, this is more than mere opinion evidence that these bikes can be built into pretty serious little "real" bikes...if you have the motivation to do so.