We do not currently know of any synonyms for operandum.
The noun operandum is defined as:
A lever, key or device installed in a operantchamber, upon which the "subject" exerts its responses. Sometimes referred to as a manipulandum. In the prototypical operant conditioning experiment, a rat (the subject) presses upon a lever (the operandum) which triggers the delivery of food (the reward or reinforcer).