Well done, the puzzle car is an Ultramobile Gazelle. One well-deserved point for you.
Hard to tell what is right about this car as the historians tell different facts. It starts with the name: the company itself was called "Deutsche Ultramobilgesellschaft", but the car was called Ultramobile (cf. the attached photos). Some sources call the car Ultramobil though (with lacking final e).
Who built it? Fersen doesn't mention anyone. Kubisch says that all were built by Boese, as do Gränz and Kirchberg. Neubauer (as you mentioned) claims Fahrzeugfabrik Eisenach up to 1906 and Boese afterwards. Ihling finally writes that most of them were built by Boese, but from 1904 to 1906 they were also built by Fahrzeugfabrik Eisenach which also built it as the "Cosmobile".
From 1906 on there was a front-engined Ultramobile following roughly the same lines. This must be the white car in the picture below. In 1908 a conventional 12 PS type appeared as a replacement. In 1909 the company was closed.
