This is a modern variant of a classic armrest design; the top is considerably more rounded than the "Vega" original and this item is not really suitable for vintage restoration work
The armrest is fixed on by being trapped against the tension ring by the tops of 2 tension hooks which pass through the slots in the side of the armrest. The armrest will then sit about 10mm - 15mm above the surface of the head, depending on the type of tension ring that is fitted to the banjo.
This armrest is designed for 8" diameter banjos (i.e. a 4" radius to the curve) but it is possible (with careful & judicious bending) to alter the radius slightly by about ½" either way.
The dimensions (all approximate) of the armrest are:
130mm long x 10mm wide x 14mm deep except at the fixing points where it is 25mm deep
The fixing slots are 19mm long x 5mm wide and the slot centres are 75mm apart and 10mm - 12mm from the top of the armrest
The hooks spacing required are quite flexible - you need 2 hooks between 60mm to 90mm apart; it will work pretty well with most measurements between those two figures though you can run into problems if the spacing between the tension hooks is less than 30mm (quite unusual)
If you need to work to precise measurements for a particular banjo, please e-mail your requirements - we can sometimes alter an armrest (at extra cost) to give different clearances from those given above.