They were common in the bars around the large factories in Michigan when I was a lad. Entire shirts of men would come off their shift and walk to the large bar across the way. To deal with this huge rush hour the bars would pour, in advance, many mugs of beer (Stroh's in Detroit) and shots of bar whiskey. These would be lined up on the bar and men would simply pick them up on their way in and take a seat.
The idea of dropping a shot glass in the mug of beer and tipping the whole thing down the hatch sprung from this tradition.
After you have two you will be saying "MakerBoiler" too.
You can order them in bars of that type to this day: you will get a mug of beer and shot of cheap whiskey. Whether you drop the shot glass in the mug after drinking the top is up to you.
Some bars in Oregon will not serve two drinks (a shot and a beer) to anyone.
I was familiar with the boilermaker. The makerboiler was a joke drink from college which switched the beer and the whiskey. I know of no one who tried one (or could survive the mug of whiskey).
I learned of those in the early 70’s, and my (now nameless) informant called them “Depthcharges.” Something about not exploding until they hit bottom...