I am referring to the 16th photo in the series of photos from the Tehran museum
http://forum.skalman.nu/viewtopic.php?f=37&t=33338#p486498. The gun in question is behind the 6 pounder AT gun. It is definitely one of the commercial Krupp 15cm (the bore is too large to be a 120mm) field howitzers similar to ones used by Argentina, Turkey (L/14), Bulgaria (M-1906), and Sweden (M/06). It would not surprise me to find that Persia at the time was yet another client of Krupp for this type of weapon. Perhaps there is someone on the forum who has close ups of this particular howitzer.