U.S. electric car startup Rivian unveiled a pickup truck and SUV last November at the 2018 Los Angeles Auto Show, but the company is known to be working on additional vehicles, one of which might have just been spotted.

Our latest spy shots were taken outside Rivian's headquarters in Plymouth, Michigan, and show what appears to be a design buck for a vehicle distinct to Rivian's R1T pickup and the related R1S SUV.

According to a source, the type of design buck is typically used to determine placement of the seats and other interior features. It provides the development team with a real-world feel for things like visibility and ergonomics, which means the shape you see here might not be representative of the final design. Furthermore, finer details like the body's surface features could also change.

Mystery model spied at Rivian's headquarters in Plymouth, Michigan - Image via S. Baldauf/SB-Medien

Mystery model spied at Rivian's headquarters in Plymouth, Michigan - Image via S. Baldauf/SB-Medien

The design buck shows a vehicle with a curved roof that contrasts with the flat design of Rivian's pickup and SUV. The belt line is also different as it rises toward the rear instead of remaining flat like on the other models.

It's possible we're looking at the rally raid-style performance vehicle Rivian CEO and founder RJ Scaringe confirmed last December. He described the vehicle as having a high ground clearance and a shorter wheelbase than the R1S, which is consistent with what we see here.

As for the specs, we can only speculate based on what we know from the R1T and R1S. Both vehicles are based on a skateboard-style platform with the battery in the floor and electric motors at each of the axles.

Rivian electric car platform

Rivian electric car platform

Rivian promises battery sizes ranging from 105 to 180 kilowatt-hours for those vehicles, the latter claimed to deliver over 400 miles of range on a single charge. Power, meanwhile, is said to range between 400 and 750 horsepower, with the output once again dependant on the battery size.

The company aims to start deliveries of the R1T in late 2020 and the R1S early the following year, with the starting prices set at $69,000 and $72,500, respectively. Production will take place at a plant in Normal, Illinois, which was formerly run by Mitsubishi.

Rivian only on Friday announced it received $700 million in its latest round of financing, with Amazon confirmed as the lead investor. The company is also reported to have inked a licensing deal of its platform with Pininfarina for an SUV from the Italian firm's new car brand.