This is the current design used an a great many convertible resonator banjos - the 4 mounting plates remain in place on the banjo but it can be played as an open-back once the 4 fixing screws have been taken out and the resonator has been removed. Older banjos of this type used a different system.
The mounting plates are approx 68mm long x 24mm wide. There are 3 slots in this mounting plate: - 2 slots (approx 16mm x 4.5mm) at the back of the plate (each about 3.5mm in from the back & ends of the plate) through which a a pair of tension hooks pass so that the plate can be bolted up against the bottom of a pair of bracket shoes and a further slot (approx 12mm x 5mm) in the centre of the plate at right-angles to the other 2 slots through which the fixing screw passes. This central slot is about 5mm - 6mm in from the front edge of the plate. As you can see from the picture, the plates are curved to sit against the outside of an 11" diameter rim. The length of the slots at the back of the plate allows for a spacing of 30mm to 53mm between the bracket shoes. This accommodates 11" banjos with 24-, 22- or 20-bracket configuration quite happily and will work for a lot of 18-bracket banjos.
It doesn't work well for 16-, 14- or 12-bracket banjos.
The wall lug is 10mm hexagonal section and fixes to the resonator rim wall with an integral stud - M5 x 0.8 pitch x 10mm long (easily shortened). It is always recommended that a suitable pilot hole be drilled before fitting these wall lugs - and it's always best to test the size of the hole first on a piece of waste wood). The wall lug is tapped M4 x 0.7 pitch in the middle (more or less)
The fixings are a type of machine screw that resembles the old "thumbscrew" but has a Philips screw recess in the head so that you can tighten it with a screwdriver rather than having to turn it by hand (although it's still possible to do it by hand if you prefer). These fixings vary a bit at the top (the upper parts varies between 12mm and 15mm long) but all have between 16mm - 18mm of thread (M4 x 0.7 pitch). These fixings fit any other wall lug with the same thread (M4 x 0.7) but it's as well to be aware that many of the older banjos of this general type have similar fixings with a different thread (M4 x 0.75 pitch) so they are not necessarily universally interchangeable, even if they look very similar.