Mules are hand-built prototypes. Cars coming off the assembly line will be different, and with better fit and finish. (I worked in a Mercedes assembly plant for 15 years so I can tell you the typical startup for a new model). Soon Tesla will start building the first Model Xs on the assembly line, assuming they plan to stick to schedule of a fall release of the Model X. Usually there should be a first run of 10 to 20 vehicles (not destined for customer sale) to work out the bugs in assembly and parts, and to balance workload between stations. Also at this time, you finalize tooling setups to get the fit and finish correct. There will usually be several test runs of the line like this, each time building a larger quantity of vehicles (again, for calibration only and not for customer sale), and operating at a quicker TAKT time (time in each station). After 4 or 5 of such test runs (usually with a week or two in between), the company will be ready to start assembling customer-grade cars. It takes several months to go through this process - at least in any Daimler plant, and I would expect Tesla to be similar. Therefore don't worry if the mules look slapped together, they don't reflect final fit & finish at all

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