Not a Tiger Moth, but
this web page has a picture of something very like you describe on a Leopard Moth.
All it consists of is a flat plate, with a few holes drilled in it, secured to a length of spring wire. As the air pressure builds up due to movement of the aircraft, the plate is forced backwards, giving a readout of the estimated speed on the graduated scale. There is even a red section, indicating when you are approaching the stall!