This is a 7,5cm Gebirgskanone 15 built by Skoda for the Austro-Hungarian Army. The Germans also used it for their mountain troops, as well as for their Infantrie Geschütz batteries.
Yes - the M15 gun had an L/15 barrel, the M28 L/18 - since the carriage and receiver were similar between the two guns the longer barrel protruded in front of the recoil assembly.