Research:

1. See this site: https://www.carbodydesign.com/2018/11/mclaren-reveals-speedt...

- Here you can read: "The first photos of the testing prototype of McLaren’s lates hypercar, which has just embarked on the real-world testing phase."

- Then you can read: "The images show the first attribute prototype, nicknamed ‘Albert’, in honor to the name of the McLaren F1 mule ‘Albert’ but also to the Albert Drive premises in which the F1 was designed."

2. Further research show that an attribute prototype is a first prototype that is built to see how parts work individually and together.

3. See this site: https://www.autoweek.com/news/a2075816/aston-martin-v8-vanta...

- Here it says: "This is an AP car—Attribute Prototype—one step before the last level of Production Prototype. But like the underskin parts, the instrument panel, switchgear, seats and door trim are all production designs, if still in development and a long way from showroom condition."

- On the same site it says: "
The test car was wired to gather data to help tune the production car that is due in U.S. showrooms later this year."

Conclusion:
An attribute prototype is the first ever built version of a prototype. Test car number one. So it´s even more than a prototype, it´s a "Testprototyp".

I think that is as close as we´re going to get.
But I´m not sure.