One small irritating feature of this design is the cross-hatched plate on the bottom that releases the lower assembly for loading film.  It is best turned using a rubber stopper or something that grips firmly.  It opens by turning to the right (CW) as viewed here.

 

This 3 element 90mm f/2.8 Tele-Lumax lens was produced in 1961 and is one of the rarest of the Periflex lenses according to Lewis. With optics produced by Enna in Germany, it has a 26 degree acceptance angle and focuses down to 2.5 ft.  More common were the 3 element 95mm f/2.8 Lumar with 25 degree angle and 6 ft minimum focusing, and the four element 95mm f/3.5 Lumax with 43 degree angle and minimum focus of 4 ft.