dxb, what is a stair-rod?

A stair rod was a thin rod, sometimes of quarter inch (6mm) diameter brass with a little knob on each end to finish it off nicely, sometimes wooden (in which case it had a right angled isosceles triangle cross section and was rather stouter, say 20mm a side, and varnished) that went across the stair at the bottom of each riser to hold the stair carpet in place. The rod itself was held in place with metal eyes, one at each end of the rod, that were screwed to the tread and riser of each step.

These days it is usual to hold the carpet with metal grippers that run across the stair screwed to the tread and riser of each step behind the carpet, hence you can't see them.

Hope that's clear.