Tesla's Elon Musk created buzz Monday night with two mysterious tweets referring to an upcoming Tesla product, boosting company shares by around 5 percent.

The caption "About time to unveil the D and something else" was attached to an image of the front of a car peeking out from a garage emblazoned with a large "D." While the tweet was unclear, it was likely alluding to an equipment upgrade rather than a brand-new model, the Wall Street Journal reported.

Musk, who is in the habit of making Tesla announcements through Twitter, told the Journal last month that the electric automaker doesn't have any models in the works besides the previously announced Model X and Model 3. The "D" mentioned in the tweet may be a driver assist feature or an all-wheel drive option for the Model S, according to CNNMoney.

Driver assist systems can slow or stop the vehicle automatically before an impending collision; sense other cars to the left and right while changing lanes; or assist with parking. An all-wheel drive version for the Model S, which has only been available in rear-wheel drive so far, would be especially important for Tesla sales in states with cold-weather climates.

Tesla's Model X is expected sometime next year and will purportedly have a roomier interior and four-wheel drive, while the less expensive Model 3 car is in development. Musk followed up his intriguing tweet with another hint, writing, "I love the Internet. Comments had me literally ROFL. No, it wasn't intentional. Glad I didn't mention the other letter!"

The Journal noted that Musk has referred to the Model X's all-wheel drive as a "dual-motor system," implying that the other letter could be "M."

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