I'm not completely sure, but I think it's a 21cm 1909 Mörser L/15 built by Rheinmetall. It could also be the 1907 model L/12 built by same, but judging from the barrel length it is the L/15... This gun was in competition with the Krupp 21cm Mörser, but was not chosen for production. Instead the Krupp gun was accepted as the 21cm Mörser (1910).
It is an interesting picture. Could this gun have been used at the front?