before you disassemble
does mirror locks up all the way
sometimes they dangle and obstruct
make sure there isn't a problem with your film holders too
take a regular unslotted holder and tape it ( masking tape ) to the back
shoot a paper negative ..
tape a sheet of waxed paper on the back and focus on it like ground glass
so you might see what is going on with the lens on B ... flip the switch
and flip up the mirror ..
ps. what does the seller say? did he/ she know about this strange problem?