Can anyone tell me what this for exactly?

[John Keen]

I have an old George Houghton and Sons Reliance open back which has an unusual set up inside the head.  The ring is set on four adjustable posts that raise or lower it thereby adjusting the action/tone/tension.  Anybody got a name for this or seen anything similar?  I'd love to know what it's actually for.

Thank you

[Richard William Ineson]

It is/was called a tone ring, Grover made one version of this appliance, and I think Essex may have made one as well. Some Windsor banjos had them installed as standard, presumably these tone rings were made by the Windsor factory. The device reduces the area of the vellum actually involved in producing the noise, so the tone of the banjo is made a bit snappier.

[marc dalmasso]

Ventilated tonering ; similar on some Windsor 's or B & M  / Abbott banjos