the pic is to small but i can see the door , I have that cabinet and the one that Treadle base mounts under the cabinet. On each of those, there is a screw on piece between the drawers, then a hinged drop down door.
to remove it kick the bellt off the flywheel, and then off the hand wheel,, right below the boobin wider at the conrer of the base is a lever push it side ways and tilt the machine to the rear.. theres two hinges under the base , you'll see those,, on the base is a set crew at each hinge, loosen those about 3 turns. The machine will slide off the hinges...
your machine has been cleaned in the past,, be very carful on these whites decals,, more so than a singer,, even quick rudding using singer oil will take off there red and green,, dish soap can be just as bad as this go off ..