If we assume that each car is road tested at the factory (which I believe was shown in the video Best of British), then let's say one test involves 15-20 miles. If a problem is noticed during that test drive, I would expect the problem to be repaired and the car retested, adding another 15-20 miles. So, it's conceivable that a car could be sent from the factory with 50 miles, if these assumptions are true and if the car required three track testings.
I ran into a guy that pit crews for Tony Stewart and others. He told me that he had toured Lotus a few years before Chapman died. He told me that at the end of the factory where the cars would leave a guy with a clipboard would take each and every car out and give it a whirl as a final inspection. I told him that they still do that and he was delighted to hear it.