I have checked online sources and now think it is a 81mm Type 97 mortar bomb because:
- It looks correct.
- It has a collar/ Fuze adapter ring which has an M24x2 Left Hand thread.
- The fuze is secured into the collar/ fuze adapter ring by a small diameter grub screw.
- This collar/ fuze adapter ring is secured into the main mortar bomb body by a small diameter grub screw.
I have checked online sources to find the differences between the Type 97 and the Type 100 mortar bomb with little success. According to Wikipedia the key differences between the Type 98 (I assume they mean Type 97) and a Type 100 are:
Type 98 Round:
Designed as the "light" HE projectile for the Type 97 mortar, it had an overall weight of about 3.3kg. The main charge consisted of approximately 0.540kg of TNT.
Type 100 Round:
Designed as a "heavy" HE projectile, it weighed from about 3kg to 3.3kg depending on the specific manufacturing variant. The main charge consisted of approximately 0.535kg of TNT.
So much for Wikipedia! This information is contradictory and unhelpful.
Another online source describes the Type 97 as follows:
“The body is of one piece streamline construction having a threaded opening in the nose to receive the fuze adapter ring. A steel booster cup screws (LH) into the lower end of the adapter ring. The booster cup is closed by a shaped aluminium container which receives the Gaine of the fuze. The Bourrelet is well-machined and has 4 grooves cut in it.”
The above information corresponds to my 81mm mortar bomb. Unfortunately I do not have the steel booster cup but the lower end of the adapter ring has female threading (approximately M35 x 2 LH) which seems to exist to receive a booster cup (See attached photos).
A different online source describes the Type 100 as follows:
“Except for minor details, this shell is similar to the Type 97 81mm HE projectile. The cup to receive the fuze Gaine is held in place in the booster cup by being fitted into a brass ring which screws into the fuze adapter ring above the booster cup.”
I conclude, based on this information, that the main difference between the Type 97 and the Type 100 mortar bomb is that the collar/ fuze adapter is different.
I would appreciate any information on this.


Regards,
Draoich