Neander K 500 (K for “Küchen-designed” engine) from 1927. This specific motor is of the non-desmo version with three valves (two inlet, one exhaust). But there were actually also desmodromic Küchen engines.
Source: Motorrad Classic 1/91 Jan./Febr. p. 13-18 and 2/91 Mar./Apr. p. 36-43
[Classification = Group 3]