I don't know that coupling but in the absence of someone who does, I would be investigating what is inside the rearmost section by removing the locknuts and pulling it off. I would expect to find some sort of collet arrangement that squeezes the shaft to grip it. You may need to loosen the nuts that tighten the stern gland to get enough movement on the shaft to bring it closer to the gearbox when you know how it goes together. You will be able to push the prop better from inside the weedhatch.
Hopefully someone with knowledge will be along soon.