The Transmitter/Filter Board (on the right in the photo) is a replacement for the Digital Transceiver Board (on the left in the photo).
It can emulate the 1200 bps AFSK AX.25 mode, making it compatible with the old transmitter, but it also supports future Fox emulation modes.
Here are the instructions to build it!
Here are the instructions to use the experimental Fox emulation mode with the new TFB.