Rich is here (but I can't remember his username )
As for what the castle mention in the mei might mean: It doesn't just say "jo", but it says "jo no shita" (in your your first pic that kanji in the middle where there seems to be s small space) which can be interpreted as "under the castle" or "in the castle's shadow"
The Bizen sig (and considering that yours doesn't have number) is probably his "private signature", in other words used not for gunto made for the military but for private purchase blades. I know yours is in Gunto mounts, but it could still have been privately purchased, not made for the military.
I think his workshop might have been near the castle, that's why he used that signature.
The sword on Rich's site is dated Dec 43, yours Oct 43. Unfortunately, Rich didn't include the date in his pictures. EDIT: Show 19 is 1944, not 1943
If anyone has Slough or other reference books: I know that an oshigata of a dated blade was printed there.
But this is just my opinion. There is hardly any information available on Nagamitsu besides what Rich already wrote on his site.