The phrase "prototype stage" refers to the initial phase of developing or creating something new. It is when a basic version or model, called a prototype, is made to test and refine the idea before moving on to full production. Full definition
The system is at prototype stage: at 1.6 kilograms, it's a bit heavy to walk around with, and remains connected to a laptop for image - processing while it runs. (newscientist.com)
Since the watch is in early prototype stage the features aren't working as expected. (windowslatest.com)
Providing they have reached the working prototype stage of development, studios can nominate themselves for a grant of between $ 5,000 and $ 50,000. (mcvuk.com)