I wonder if thats 40 caroms in cowboy? or 5 racks of 8 ball?
The caption mentions (and shows) cowboy pool: you play with 1, 3, 5 and can score with carom, bouncing the cue off 2 other balls, pocketing also scores the number on the ball till 90; till 100 only caroms score. to get 101 and win, you must scratch off the 1. Fun game, my favorite when not paying for each rack.
They are almost certainly talking about 14:1 continuous which was far and away the most popular pocket billiards game in the 1930s,40s, and 50s.
40 is pretty good but great players regularly run over 100 and John Schmidt just ran 600+
Michelson was playing billiards on a table without pockets.
I agree that Michelson wasn’t playing 3-cushion because not even Willie Hoppe (best player of that era) had runs 10 or 12 on a daily basis.
Balkline was still popular as an amateur game in the 1920s, so Michelson could have been playing one of the balkline variants rather than straight rail.
Level of difficulty in billiards ...
Straight Rail < Balkline < Cushion Caroms < 3-Cushion
In the game of cowboy, which is sort of a novelty game that is intentionally quirky, you can score points merely by making a carom.
Here's an example I got by Google image searching: https://i.makeagif.com/media/10-30-2015/ODVWHH.gif