Many early mini #18's came without a sheath. A "cover" was provided often but no sheath. Of course, they also did not come with the spear shaft either. Having said that, the combo is fairly uncommon and worth the dough. 300 minis sounds like an awful lot. I have owned several and "estimate" is using the word pretty liberally. Joe may have some data on actual numbers but I venture to say less...way less.
Nice rig!
Best, Capt. Chris