The Mauser Einspurauto (Einspur-Auto - single-track) was first conceptualised by Gustav Winkler in 1921, with production models appearing in 1924 powered by the same BMW M2B15 engine found in the Helios and the Victoria. These were probably built by Atlantic. Mauser sold the production rights to a French concern where it was produced as the Monotrace. The main visible difference between the two is that the Mauser stabiliser wheels were retractable, but on the Monotrace they were not.
There is an example of the French version at Musée Henri Martre
The Mauser arms factory was established in 1874. Under the terms of the Versailles Treaty they were no longer permitted to make weapson, so automobiles were one of the ventures they embarked upon. In addition to the Monotrace they built cars and trucks until 1927.
Single Track Runabout.
If our recollections are correct, the Mauser is the revival of an idea put forward in America several years ago. The machine is an ordinary type motor cycle with rather lengthened wheelbase, driven by a B.M.W. engine, and having a car type body built on this frame. There are two seats in tandem, and in addition to the two ordinary wheels a central axle with a small diameter wheel at each end. When standing, or running slowly, the axle can be dropped, allowing the wheels to come in contact with the road, but when running normally the axle is raised, and the machine runs just like an ordinary motor cycle. The flat twin engine, being placed fore and aft in the frame, is particulaidy well suited for this type of motor cycle. It is carried under the floor-boards, beneath the passenger's feet, and is invisible from outside.
There are openings in the body to allow of a cooling draught.
The Motor Cycle, October 6th, 1921. p417
Albamobil
This machine bears a marked resemblance to the Mauser, and it is thought to be a prototype from around 1920. It uses the same system to activate the side wheels as the Mauser, and has a registration plate from Brandenburg.
Sources: François-Marie Dumas, The Motor Cycle