This is Kees K5BCQ 6x LPF/BPF board showing one LPF plug-in. The LPF parts were removed from the uBITX and moved to the plug-in board. This is easy with a hot air rework station like the ones all over eBay for about US$50. The relay removal is also “relatively ” easy with no pulled pads (using the large square tip which heats 4 pins at a time).
The plug-in footprint is the same as QRP Labs uses on their LPFs/BPFs. Kees mounted the pins on the other side of the board because it reduces the overall height. You can solder the coax to the SMA connector footprint or use SMA connectors. Many options.
The photo below shows the four transplanted filters from the main µBITx board and the main board LPF corner that is now stripped bare.
He moved it to a new thread, the link you have is now dead. Try https://groups.io/g/BITX20/topic/25164601 instead.
I fixed the link …