thumbpiece

[thuhm-pees]

noun

a lever over the hinge on the lid of a tankard, pressed by the thumb to raise the lid.

Origin of thumbpiece

First recorded in 1750–60; thumb + piece

Also called billet.

A thumbpiece must be sewn on the bag at the hasp to open it by.
Scientific American Supplement, No. 561, October 2, 1886
Various
He knows it will squirt if he touches the thumbpiece, but it is as liable to hit him in the face, or wet his feet as anything.
Peck's Uncle Ike and The Red Headed Boy
George W. Peck
Above the handle, and attached to the lid, a metal flange or thumbpiece.
Europe After 8:15
H. L. Mencken, George Jean Nathan and Willard Huntington Wright
Grasp the handle, press your thumb on the thumbpiece—and presto, the lid heaves up.
Europe After 8:15
H. L. Mencken, George Jean Nathan and Willard Huntington Wright
They were almost upon him when Kalvar Dard pressed in the thumbpiece of the bomb; they were clutching at him when he released it.
Genesis
H. Beam Piper