This is a modern variant of a classic armrest design; the top is flat with a rounded-off corner on the outside edge.
This item isn't really suitable for vintage restoration work.
The armrest is fixed on by the single leg being trapped against 2 tension hooks. There is a flat fixing bar which passes behind the hooks and the armrest is fixed to the bar with a fixing screw - the fixings are included with every armrest.
The armrest will then sit reasonably comfortably on top of the tension ring - or a little higher, if preferred, depending on the exact configuration of the tension ring & brackets.
This armrest is designed for 11" diameter banjos (i.e. a 5 ½" radius to the curve) and it is quite difficult to fit it to a banjo which has a radius much different from 5 ½" - the leeway is perhaps 3/16" at most.
The dimensions (all approximate) of the armrest are:
180mm long x 20mm wide x 18mm deep except at the fixing points where it is 38mm deep
There are 2 pairs of fixing slots in the leg: one at approx 39mm centres (for 24 bracket banjos) and one at 50mm centres (for 18 bracket banjos). As there is often a little bit of "wiggle-room" on tension hooks, this armrest will fit a great many modern banjos.
If you need to work to precise measurements for a particular banjo, please e-mail your requirements - we can sometimes alter an armrest to give different clearances from those given above.