I think some areas actually had dc power in those days. A transformer set would have been useless. The low cost probably made it catch on where it wasnt needed.
I think "hot compared to what" would have been another issue. It wouldnt have been like today where the electrical box is clearly grounded, and the neutral is at the same potential as ground. The chassis is clearly hot compared to all sorts of stuff around you, at least if the plug is in a certain way. Then, a lot of houses that were wired in the teens or 20s had 110v drops from the distribution system. Nothing was grounded. They used to fuse the neutral. That sounds insane today, because the neutral fuse could blow leaving the hot hot. Then, though the neutral was coming from the distribution system and could be a little bit hot. Probably not as hot as the hot. There could be some voltage difference between the neutral and say a water pipe in the house.
110 doesnt usually bite that bad, and it was fairly common to have 400v (or more) floating around in a transformer set of that period. I doubt those sets were considered the most dangerous at the time.
Also, it was probably assumed you werent going to lick it or take it in the bathtub with you.
Today, that would be the content of the instructions. Back then, they probably told you how to use the set.
John