Im fairly certain that its Walrus, Ive got some old (pre-1930s) documentation from Winchester, they were having some issues getting Sea Horse hide, and tried cow hide but found it unsuitable.

Sea horse was the period name for Walrus.

This was confirmed by a good friend of mine that used to work at Winchester; also, as late as the early 80s they still had a couple of remaining walrus hides to make wheels.