I went to install the C adapter and did not find any Y or W. There is a R and G. Is XFMR-R and XFMR-C another label for Y and W?
XFMR is short for transformer, and they can’t be used for Y and W wires, I would think that kills the adapter. There’s got to be some other place where you can find the Y and W terminals, although there may also be wire nut connections instead.
Can you see where the wire bundle from the thermostat comes up to the furnace and post a picture of where the wires are currently connected?
This is a fan control board. Your air handler likely uses point to point wiring and not a unified control board. Show multiple wide angle images of the inside of your air handler and anything that looks like the wires you have behind your thermostat. Also, if you have a model number or a schematic, that will be extremely helpful.