The interpreter is part of the binary yes. I have reversed what was available here: http://squoze.net/B/ and written my own B compiler that generates the same threaded code as well: https://github.com/aap/b
I actually didn't know about n.out before. It wasn't mentioned in the v1 manual so they must have gotten rid of it by then. However it seems that it is just a symbol table that's generated by the assembler. B code is eventually also passed to the assembler so it would just contain whatever symbols were in the assembly i suppose.
what do you suppose the b command put in n.out