It looks very much like a mortar.
"The Tiran 5 conversion includes: hollow cast turret with numerous IDF details cast in, empty bustle cast to the turret, IDF modified loaders and drivers hatch, 105mm gun tube. 60mm mortar and spare round box, commanders and loaders m.g. mounts, correct machine guns, .30 caliber and correct ammo boxes. "
According to
https://www.globalsecurity.org/military/world/israel/tiran.htm the 60 mm mortar came with the Tiran 5.
I hope some Tiran expert can clarify this.
There is build review over at Britmodeller:
http://www.britmodeller.com/forums/index.php?/topic/235024472-idf-tiran-4-135/ "The build then moves onto the turret, with the single piece upper section being fitted with the hatch rings, internal co-axial machine gun, various brackets, stowage eyes, sight doors, and hand rails before the gunners hatch, which is made up from no less than twelve parts, is attached, along with a sight. The much simpler, four part commanders hatch is glued into position, as is the mantlet cover, three piece 60mm mortar, two jerry cans, small two piece storage box, three piece large storage box, two, three piece aerial bases, and ten piece rear stowage basket. "
/ Robin