The throttle is a Domino 0-5kOhm model. It is a potentiometer that varies from zero at no throttle to 5kOhm at full throttle.
The more common throttle used on electric motorcycle conversions is the Magura throttle, which is half the price of the Domino. But the review by Ted Dillard in The Electric Chronicles convinced me to try the Domino.
Domino 0-5 kOhm Throttle
The throttle comes with a left grip to match the right, plus connectors.