While Tesla has now thousands of vehicles equipped with its latest Autopilot hardware on the road today, only a few prototypes in its captive fleet have a software build capable of level 4 or 5 self-driving installed.

In its latest Disengagement of Autonomous Mode report with the California DMV, the automaker confirmed that it operated 4 of those prototypes last year and that they accumulated 550 autonomous miles in 2016.

It compares to the 1.3 billion miles of data they accumulated with the Autopilot program as of November and the roughly 300 million miles driven on Autopilot, which is a level 2 autonomous system.

Companies testing fully autonomous vehicles on public roads in California have to issue a report on the disengagement events with the vehicles under testing.

Tesla issued its report for 2016 and confirmed that it drove 550 autonomous miles in 2016 with 168 disengagement events.

Interestingly, all of Tesla’s prototypes that drove on California’s public roads last year were Model X SUVs based on the VINs listed in the report:tesla-autonomus-miles-driven

The fourth letter in the VIN represents the model (S or X). There are apparently no Model S in the captive fleet for testing in California.

Also interesting, Tesla apparently mostly stopped testing fully autonomous vehicles in November after first demonstrating the technology in October, when they completed most of their autonomous mileage. They completely stopped testing the vehicles in December.

Update: A source familiar with the program told Electrek that the autonomous mileage was mostly accumulated for the production of the demonstration videos that they published. It would also explain why they were only using the Model X since it was the vehicle used in the videos. The Autopilot team also tests vehicles outside of California’s public roads and mainly focuses on using data collected through its fleet.

Of course, the fully self-driving effort is part of Tesla’s ‘Enhanced Autopilot’ program and the company focused on making the software build for the system available to the fleet over the past few months. It was supposed to be available in December, but it was ultimately pushed to the broader fleet late January.

The Autopilot team could have been focused on the ‘Enhanced Autopilot’ update instead of the special fully autonomous software in those 4 Model X prototypes.

Tesla is currently building its second generation Autopilot using the new hardware suite and its ‘Tesla vision’ image processing system. All vehicles built since October 2016 are equipped with the technology and upcoming software updates will gradually improve the performance and introduce new autonomous features leading to fully autonomy.

CEO Elon Musk said that he expects Tesla to have a software build using the current hardware suite capable of an autonomous coast-to-coast drive by the end of the year.

Here’s the report: