Andy,

You are correct in that while the IJN used the Nambu type 14 and other pistols, I have found no mention of special marks or even reserved serial number blocks. From what I read, the type 14 was used by officers and NCOs of the IJN as well as the Special Naval Landing Forces or Rikusentai.

Wikipedia says this about the ammo:

"The Japanese Army cartridges in 8 mm Nambu have no markings on the headstamp unlike the Japanese Navy cartridges"

Maybe they are full of beans, but it could be interesting to know.

Dave