Does anyone know what Coupling this is please?

[Dougal T]

It needs to come off and is being the usual pain, almost certainly going to have to cut it off, but would really like to know what will be left of the shaft so an appropriate replacement is ready. There is a 41mm nut on the front locked with a grub screw that needs a cut down allen key to remove since its in a recess.
Just seen the woodruff key at the back, so I guess that at least some of the shaft inside is straight....

Your best guess will be really handy.