Thread by @UAWeapons

After spending a whole day on this we managed to establish the most likely path of these Japanese mortar bombs to Ukraine: as we said initially, they were in fact provided by a third country - in this case it was the United States of America.

https://twitter.com/UAWeapons/status/1546141571552272384

Japan is known for its strict lethal weapons export policy - but it was not always so.
In 1952 Japan was again allowed to manufacture weapons after WW2 and in the same year the US ordered a huge batch of 265,000 60mm mortar bombs from the Takakura Plant (Daido Steel).

These mortars bombs were ordered for the US and allied forces deployed in Korea and it seems after the end of the conflict at least a part of them were brought back to America.

This theory perfectly explains the use of Kanji characters, which are quite rare today - in the early 50s they were still used, which is confirmed by their presence on other types of weapons.
Here is a crate of Japanese M28A2 Bazooka rockets with the same characters.

Another piece of evidence for the Japanese origin of these bombs is the structure of the lot number.
LOT "J-NT-2-9*" matches the JSDF naming convention, where J used with license-built weapons, NT for a factory code, 2 for a "central number" and 9* for a serial number.

So the Taiwanese officials are correct to deny that the mortar bombs are from their stocks and they pre-date the current Japanese export policy.
Indeed a curious story stretching back many years; and another case of the US providing weapons from the most obscure sources!

Thanks everyone who helped to dig into this case (@SHIMASHObio and others)!