Least significant nixie digit drive shows Even and 2/3 lines active, thus the 2 cathode is to be on.
Now that I have assembled the #3 line to 32 line output expander board, there is no more excuse for procrastination. So I connected it up to the #Quick and simple STM8 board with TCXO, rebuilt the software to drive the 74HC595 shift registers instead of a TM1637 and flashed the STM8 MCU. Since the DS3231 TCXO keeps time even in the absence of power, I know what to expect the display to show. I have connected LEDs only to the least significant digit (minutes) but from previous testing, the whole shift register chain should work.
At the top left is the power adaptor board which regulates the input voltage from a 12V wall wart power supply via the barrel socket to 5V and 3.3V. Here it's used only to tap into the 12V as there is a 5V LD1117 regulator on board. The adaptor is a dollar or two ordered online. The board is plugged into a breadboard which here only provides connection points. It was previously used with the Arduino Uno for earlier testing.
The third board, not shown, is an off-the-shelf 12V to 200V (adjustable) boost converter. All of the add-on boards to the thermometer will be powered by the 12V wall wart. The expected draw from the power supply is around 200-300 mA, well within the capability of the many spare wall warts in my spares hoard.