Partly solved: One of the new displays (marked 2004A-1 with "MH" on the driver board) was shorted to the mechanical display attachment tabs. The tabs are in a new position where they can touch the 2-pin display power connector. This board was at address 0x3F, but once I got it working it had a small group of pixels in the center of row 0 and row 1 that are bad.
An identical board, from the same vendor in the same order, was at address 0x27. I guess I'm going to have to do an I2C address search during setup() if I want to use these as plug and play.
One remaining issue: some of the boards are plug and play, in the sense that I can unplug one and replace with another and the display comes alive with the current messages. Others require that the Arduino be reset in order for the messages to appear.
All of the above is based on using the code above, the "out of date" stuff. I'm still working on understanding all the functions in the HD4470 library.