Pot Bolts

Traditional Banjos have a perch pole (an adjustable wooden dowel in the pot) or adjustment rods (adjustable metal rods) to stop the stings pulling the pot or rim out of shape, some of the better built full size Zither Banjos also have a perch pole but not the smaller Zither Banjos and Mandolins.Instead there are two small bolts attached to the tension hoop one at the neck side and one at the tail side. I don't know what they're called but I'll call them Pot Bolts because of their function.

The bolts like the tension bolts have a hexagonal head the same size as the tension bolts. I found they fitted a  6mm socket, so being imperial they're probably 1/4". Obviously I couldn't fit a socket between the edge of the pot and the metal ring so I had to make a trip to Halfords for a 6mm wrench.

Loosening the the bolt moves the heads out towards the wall of the pot pressing outwards and helping support the inside of the curve against the pull of the strings.

Some zither instruments have brackets at 12 and 6 o'clock on the tension hoop which would probably preserve the shape of the pot without the user having to make adjustments. I'd guess with less scope for user error those instruments will've probably fared better with the passage of time. With the pot bolts adjusted the instrument was ready for a bridge and some strings.