Not much to go by since the carriage is hidden. But I believe this to be a 10cm Kanone 1899. The postcard was sent from Juterbog in Oct 1916. Any assistance appreciated to confirm or deny my identification...
If you notice the trunnion for the gun, the supporting carriage is made of bar stock and is open in design. The 10cm Kanone 1899 and Belgium 12cm has sheet metal up to the trunnion. The piece appears to be a German 12cm Schwere Kanone:
Charlie and Ralph, thank you for your quick replies. I thought it was too small of a tube for 15cm, but I wasn't comfortable with the 10cm either - which is why I sought confirmation here. I hadn't thought of 12cm - I skipped right over those in Kosar because I thought they would be too old. Always a mistake with Fussartillerie, especially since this is Juterbog.
Ralph, I believe you are spot on with the trunnions; I overlooked that detail. After reviewing your website and going back to Kosar and Jager, I agree this appears to be a schwere 12cm Kanone M1880.