Hi Ray,
Interesting job, sure enough.
I suspect the still-soft stitched scabbard has been placed on a slim former (possibly the sword itself wrapped in greased paper)while the outside was finished. Then the goop, whatever it is, tipped in and swilled about then tipped out and the scabbard left to drip.
I'm not saying this is the way it was done, just how I would approach it.
Many old glues, based in fish and mammal (horse and cow) proteins, cross-link and form a rigid film after they dry in air. Fish glue goes very stiff and solid. Have you seen a dried fish? like a rock.
After the fish glue hardens it can be varnished to protect it from moisture.
Dunno............
Cheers,
Karl