Knob to replace plastic cap on adjustment lever of wall registers, e.g. Signature Hardware '4" x 8" Mission Solid Brass Wall Register' (https://www.signaturehardware.com/mission-style-solid-brass-wall-register-with-louvers.html).

Dimensions of knob: 20mm by 11mm, height 12mm. Unlike the original, it provides enough friction to open vent just half-way. Fits tightly onto the 10.75mm x 1.85mm metal tab (pull off original plastic cap if you haven't already lost it).