This photo is about the earliest instance of the herald I found in a quick troll through the DPL site - September 1941.
October 1941:
It appears that 41, 20, and 455 got the herald by 1942.
I didn't see any in 1940.
I didn't see any photos prior to 1941-1942 of K27s on the line, either. I mention that to make a guess: that the herald is tied to the arrival of the K27s. Perhaps it was designed for the first K27 sold to the RGS, then applied to other locomotives and cabooses as they needed paint?
JAC