The phrase "a prototype that was" is correct and usable in written English.
It can be used when referring to a specific prototype that existed or was created in the past.
Example: "The engineers presented a prototype that was developed to test the new technology."
Alternatives: "a prototype which was" or "a prototype that existed".
In their office shop, they built a prototype that was smoothly opened by a motor instead of being jerked open by a hand crank.
Last week, Mr. Olivares unveiled his Hollis side table there, a piece introduced in 2010 as a prototype that was now being readied for the market.
Although it was a prototype that was not for sale, on the designer's Web site, Mr. Gilad found something he liked that was available: Vase Space, a side table with three removable wooden vases.
Origami TV Remote may look cool, but it's really just a prototype that was developed during a workshop.
A proof of principle is given by observing a prototype that was milled into a copper bar.
Then we just started working on a prototype that was just functional enough to put in users' hands and get feedback.
Engineers from Sony will exhibit a prototype that is, at 13 inches in diagonal, the world's biggest.
An inventor in Phoenix, Arizona has cobbled together a prototype that is, frankly, a bit of a letdown – it's a mini hovercraft with rubbish stabilisers, but one that suggests the technology could be acceptably refined.
Sony is launching its Morpheus VR headset for PlayStation 4 in the first half of that year, having recently shown off a prototype that is close to a final consumer version.
Sony also plans to launch its Morpheus VR headset for PlayStation 4 in the first half of next year, having recently shown off a prototype that is close to a final consumer version.
With stores that average 80,000 square feet, compared with 150,000 square feet for Target and 200,000 for Wal-Mart, Kmart could come up with a prototype that is small but efficient.
