There were a great many improvised munitions in both China and the Philippines, built by resistance fighters in the areas. China has at least one museum dedicated to them and their equipment. Many of these used ordnance developed or found in the areas, modified to utilize captured Japanese equipment. Interesting that this item shares some features of French 5cm mortars.
The MEIU folks at the time did a pretty good job of trying to document everything that they found in the different locations, putting out the MEIU Reports. These reports were later consolidated into the Blue Book, which weeded out (dropped) some items that were not actual adopted items or items seen in significant numbers. The Blue Book was then revised with a second volume, and then the volumes were later consolidated and turned into OPNAV 30-3M, one of the best US documents done on Japanese ordnance. If you have many of the original reports, followed by the Blue Books (rare) and the OPNAV you can track the history of the items, see how the intell was processed for mass consumption, and note what items didn't make the final documents. Best guess is that this may be the case with your mystery mortar.
