What Delberthot said...
Although, for some inexplicable reason, they are, these days, universally called thumbrests, even when installed below...you just try playing with your thumb down there. Finger rest! Additionally, the early single-screw rests were made from maple, raw, or painted black. The two-screw version was briefly on the SCPB before the split-coil debuted mid-57...
[/geek]