This was taken just after a heavy rain, when the silt had the effect of making the very faint letters a little more readable. I am frustrated by my inability to read the date. Even feeling it with my fingers, I can make out only the initial “19” and nothing else.
This stamp is located on the east side of Regent Street, the 400 block, between Kalamazoo and Elizabeth. The style matches the 1918 Department of Public Works imprints I found previously, and is distinct from the 1944 DPW stamp and some other 1940s DPW stamps I have seen on my walks. I hope in time to find some stamps eventually that will narrow down when they switched styles, so I can give a latest possible date for this stamp.
Update 12/10/20: the date has become visible! See my update.