I guess I didn't need to ask the above questions. But for those interested:

I forgot that I had a copy of Cornell Publications reproduction 1929 Steyr catalogue with an illustrated parts list...and the rifle in question coincidentally was made in 1929.

According to the illustrated parts list the grip cap on the "British Sporting Pattern" rifle is made from "Vulcanite" (German: Hartgummi?).

According to various sources Vulcanite (also known as Ebonite) is a hard black rubber made by the vulcanization of rubber with large amounts of sulphur. It may be cut and polished, and is used for many small articles, as combs and buttons, and for insulating material in electric apparatus. During the Victorian era, the ebony tree of southern Asia yielded very popular, dark-coloured heartwood and was prized for cabinetwork and piano keys. Vulcanite resembled ebony, hence its popularity. Today, the terms Ebonite and Vulcanite are used interchangeably. Vulcanization is a chemical reaction of sulfur with rubber that causes cross-linking of polymer chains; it increases strength and resiliency. Vulcanite was patented by Charles Goodyear in 1846.

The catalogue also shows a picture of my cleaning rod...4 piece for carbine (and in my case, 5 piece for rifle).

In the same catalogue the grip caps on regular M-S carbines and rifles (without the hollow grips)are listed as being made of horn, but this late (1929) and most of mine have steel grip caps.