This is an older 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. More modern banjos of this type used a different system - please see the product above this one.
The mounting plates are approx 61mm long x 23mm wide. There are 3 slots in this mounting plate: - 2 slots (approx 12mm 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 7mm 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 2.5mm - 3mm 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 46mm between the bracket shoes. This accommodates 11" banjos with 24-, 22- or 20-bracket configuration quite happily and can just about be made to work (with a little judicious filing) for some 18-bracket banjos. It doesn't work at all for 16-, 14- or 12-bracket banjos.
The captive thread is approx 7mm round section and fixes into the top of the resonator rim wall - ideally by drilling a slightly oversize hole and gluing it in; it can simply be hammered into a slightly undersize hole but this method is not recommended. The captive thread is about 10mm long (easily shortened at need) and has a coarse thread, very close to the old 3/16" BSW. It is always recommended that a suitable test hole be drilled in a bit of waste wood before fitting these captive threads - they are designed to fit flush to the surface of the wood which can be a bit tricky to gauge.
The fixings are a type of machine screw that resembles the old "thumbscrew" but has a Philips screwdriver 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 are approx 26mm long; the top part is approx 12mm x 10mm diameter, the threaded part is approx 14mm long. These fixings don't seem to fit anything other than their own captive threads apart from a very few vintage British banjos which have an almost identical thread.