My guess is the hubs are set up to accommodate a Phil Wood disc brake. The ingenious design did not require any special anchor brackets on the frame. I have one on my Mercian tandem (rear). I have it connected to a old Campy barcons to use as a drag brake to help limit speed on long descents. You don’t need a whole lot of help from the drag brake to significantly reduce the fade from the rim brakes ( the tandem has Mafac cantilevers in that application.) The Phil disc brakes are long gone from production, but when I was restoring my Tandem, the guys at Phil were very accommodating and threaded a cassette hub to mount the brake.