Gentlemen,
We have a winner. Nomadr: Your'e absolutely right. Did some searches on MG13 and found a good description + photo on the following links:
http://www.military-collections.com/weapons.html
http://www.dasheer.org.uk/machine_guns.htm
Note that many sources claim that the MG13 was taken out of service in 1934. These images, dating from the 1935-1937 era proof that this is not entirely true and show these were in use, but most probably just as training weapons, well into the late 30s. Thus matching Nomadr's 1939 date.