The phrase "a working proof" is correct and usable in written English.
It can be used in contexts where you are referring to a proof that is functional or effective in demonstrating a concept or theorem.
Example: "The mathematician presented a working proof of the theorem that satisfied all the criteria set by the committee."
Alternatives: "a valid proof" or "an effective proof".
The one on display here is unusual, rare and interesting because it is a working proof, an early, probably one-off print in which we see the artist working out the basic color scheme.
Finally, we have presented here a working proof that tuning in simple target detection is a useful scale for measuring the degree of attentional focus in brain activity.
What happened to all of that money, harvested from the credulous in a decent working proof of how most wealth remains uncorrelated to intelligence?
"In order to be eligible for an Apple Security Bounty, researchers are required to provide a clear report and working proof of concept.
The determination and resolve of all political leaders in the face of this brutal act is working proof of a unity of purpose.
The paper treats the litter problem, the architecting phase, and will show innovative technical details, including a working integrated result: a proof of principle of the robot.
In one sense, this is just practical proof of a working assumption.
For example, a mathematician working on a proof regards a conjecture as plausible if it "has some chances of success" (Polya 1954 (v. 2): 148).
A neighbor says he asked a man working there for proof he had a permit to take down the painting.
In this work, proof of a conceptual and innovative design of an endoscopic piezoelectric tactile sensor is presented.
The work started with a single developer working on a proof-of-concept for 4 months, at the end of which we had a basic but functional prototype, proving the feasibility of the project.
