Here’s another shot at this one:
I recently got a copy of George Markham’s “Japanese Infantry Weapons of World War Two”, and he describes something called the “Type 98 mortar - kuhachi shiki keihaku gehiko”, which seems to correspond to your pictures and description. He doesn’t have any pictures, but I think I’ve found one on the net (below). His description goes as follows:
“The 50mm Type 98 mortar, introduced in 1938, fired a large stick bomb rather than a self-propelling shell or conventional finned bomb. The large cubical metal-case charge was mounted on the tip of a long wooden rod that went down the barrel. The metal rod tip rested on the propellant charge. Incremental charges, or the attachment of a regulator to the muzzle to control the bore intrusion of the rod, varied the range of the grenade. The barrel was raised by a sliding collar attached to the bipod and locked at the desired point; traverse merely entailed unscrewing the wing nuts holding the bipod to the baseplate and sliding the feet to the desired position. A simple friction-ignition firing unit was used. But the weapon was very crude and probably not mas-produced until the 1940s.”
He seems to be describing this item:
Although the warhead in your pictures seems to be off the standard Type 91 hand grenade, it’s likely that all of these bombs were threaded for the same thread size, and by the time they were producing these, they needed all they could get, and yesterday; substituting a grenade body for the standard warhead shown in the picture above would have also added to the range. The base of the rod seems very close to what I remember of your pictures as well. Hope this helps.