40 years- when most power supplies were still linear? Not believable. Why 54 volts? Not 55, not 50. Also suspicious.
40 years ago smps do exists, nothing suspicious about it. Computer smps for example. IBM XT PC, I know because I worked at IBM back in those days, and repaired many units, along with mainboards, not like today where we just replace parts... Comodore Amiga are also an example of using smps, the 500 model uses an external smps, this model come out maybe 3-4 years after IBM if I recall correctly. So smps do exist back then, not so common as today's days.
concerning 54V, it depends on smps design some start to operate at around 44v some a little less, but for stability a little higher voltage is required. Of course everyone is responsible for their actions, meaning if you decide to try it and burn the device don't blame me. I do it anytime for many years with zero failures. even with those cheap chinese dvd players with shitty PS. And although for most of you operating a scope with DC power is not a concern, every manufacturer should consider this feature, because we live in a time where portable devices are a must these days and there's not much offer in this class of equipments .
And btw, for those amateurs who will find this amazing, beware that dealing with 54V DC could kill you or burn you.