Manufactured by Bosch Stockdorfer Motorenwerke AG in Stockdorf near Munich.
Bosch began building Douglas engines under licence after the First World War. These engines were supplied to HMB (Hirsch), KRS , Linsner, SBD and Bison.
Karü (KR) also built a version of the Bosch-Douglas which was supplied to Astra, Bayern and Bravis. The SMW motorcycles built by Karü also had these engines.
Martin Stolle of BFW based the first BMW engine, the M2B15, on the Bosch-Douglas.
(Information above requires revision.)
Sources: wikipedia.nl, et al.