Well, here we go: It can be assumed that the author of the fair report in "lastauto-omnibus" used the press releases of the manufacturers, without questioning them. Which was and is common practice.
The cab was developed and built by a supplier (you have already mentioned his name), this is confirmed by various sources. This one-man cabin was also used in a modified form for other applications. The big manufacturers also usually had no capacity for such smallest series.
To the chassis: I have a strong suspicion that a mistake has been made when setting up the exhibition stand: Henschel-Miixer = BM-chassis.
The exhibition vehicle, the puzzle truck, had a subframe, to be able to exchange the mixer for other superstructures.
The Henschel BM-chassis was a very special, unique construction. It was the only chassis on the market, which was able to compensate the dynamic forces of a mixing drum, rotating during the drive, without using any subframe or additional reinforcements. There was, for example, an additional spring on the left side to balance the wandering centre of gravity when the drum rotates clockwise.
So, Henschel was number one of mixer-trucks in Germany until the end. At the best of times, they sold more mixer chassis, than all other manufacturers combined.
I add three photos: first: the puzzle-truck with subframe for changing bodies, second and third: Henschel BM-chassis. I think the difference is clear.
So, please add the coachbuilder and the possible alternative to the BM-chassis and of course manufactorer of the interchangeable frame.