Despite what some internet sources say, I am pretty sure that the puzzle car is not "mixte" - v. Frankenberg may well be wrong too, but I don't think so. What is clear to me is that there is no sign that it is other than an electric car.
In "The Lost Causes of Motoring" (Europe Vol 1) Lord Montagu, writing from Michael Sedgwick's research, says:" That early British entrepreneur, E.W. Hart, tried to introduce the system to our islands, and ran a four-wheel drive model in the ACGBI's Electric Car Trials in November 1900. Alas, Hart had a genius for backing the wrong horses, and the car he had christened La Toujours Contente turned out to be just that, achieving nothing."
To labour the point, a "mixte" would not have qualified for those trials, and I think they did not appear until 1902 anyway.