I own a pistol, that I built to use in parades, where blank fire is allowed. It is a grand mixture of parts from several kit guns, and some hand made parts. The barrel is a round .68 cal. smoothbore, plainly stamped "Japan" on the left rear of the breech. I have examined this barrel every which way, short of having it X-rayed, and I find no signs of breech threads. I, in fact find no signs of a breech plug. My best guess is this barrel/breechplug is all one piece. I did a repair on a pistol that had a barrel that was outwardly the same as mine, but when I pulled the barrel, it had a standard breech plug threaded into it. So, I think there were a few Japanese guns made without breech threads, but they probably had no breech plug either.
Hungry Horse