I walked a different route from usual which took me to the east side of South Clemens Avenue between Kalamazoo and Prospect, instead of the west side where I more often walk. And look what I found there.
So the oddly placed 1921 DPW stamp on Regent Street isn’t unique or a misfire after all. There are stamps from both before and after this one that are more conventionally placed. There is even another one from 1924. So my new hypothesis is that there was one particular foreman in the 1920s who liked it this way.
Too bad about the crack; it’s otherwise very clear. It also gives me my new latest date for DPW stamps marking the month as well as the year.