A BB605 came available at a reasonable price - me thinks "lesser model, I'll swap the neck onto my 615".
Conversation with the authority on all things Yamaha, @AndyTravis and I find out that the 605 is 24 frets, 615 has 21.
The 605 arrives today - nice condition but issues with the output - NONE!!. Message to my electronics guy - waiting for a reply.