I don't know what the neck joint is on these. They were well made Japanese instruments in the 70's and they have a following. There is a Facebook page for them too if you want to check that out. I've worked on them before but it's been maybe 5 years since I had one on my bench.
There were a number of high-end Japanese Martin clones and some used "pinned" aka "dowelled" neck joints. The Hashimotos were solid wood IIRC.
What's interesting to me is one way to reset these if this is not a dovetail and if it needs a neck reset is to slip the neck block intentionally. It's called in Michigan a California neck reset. In California they call it a Michigan neck reset.
No one wants to own the practice because it's a poor practice that often damages the instrument and caused permanent distortion of the guitars geometry.
I have to wonder is this is damage and if it's actual damage it's a very odd way for a guitar to be damaged. Or, conversely if it needed a neck reset, and it likely would and someone tried to do a California (no offense intended to my California friends
) neck rest by slipping the neck block at the back and things did not result as intended.....Would love to have a view of what's happening on the inside.
As for restoring the body/neck geometry as Chris suggested that may not be possible if my hunch, that it needs a neck reset is correct. If so it would need the neck removed, refitted, perhaps converted to a butt joint if it was pinned/doweled and then the geometry can be restored correctly. Big job and stuff tends to happen along the way.
It was also sadly finished with the neck on so there will be extensive touch up required as the finish will have to be cut through to remove the neck.