More research suggests that the Asanuma King has triple extension to 20″, which is a darned good thing. Also, while the existing “back” looks very poorly adapted, it seems that both clips are in place to hold it. If my thinking’s right, that means the previous owner cobbled the Graflex onto the original plate back. If that’s true, I should be able to slide the plate back out and either use it directly or as a pattern for the 4×5 spring back. And I think the springs from the Graflex back will be usable for the new spring back, which means less metalwork for me.
This is all pending receipt and inspection, but the photos are pretty encouraging!