From the website:
The German Feldgendarmerie force had its origins in the 19th century, with most German states forming military units for policing duties. This was continued by the Imperial German Army during World War I when a large force was formed for all policing tasks behind the front lines, although this was disbanded during the interwar period. Reformed in 1939 from experienced army NCOs and civil gendarme, Feldgendarmerie units were attached to armies by battalion but would then be broken down into smaller units for deployment.
Utilising the tough Bedford chassis, the British Home Guard use their ingenuity to fight the enemy with the new Bedford OY SP 3-pdr.
The Bedford OY was a militarised version of the Bedford O series lorry capable of carrying three tonnes. A Hotchkiss 3-pdr naval gun mounted in the rear on a pedestal turned it into a self-propelled gun.
German Feldgendarme Motorcycle Team Source
Bedford OY SP 3-pdr Source