It is probably one of the more stable variations of celluloid, looks like your Ka-Bar there is 1930-1940 and plastics were quite a bit better by then but they feel like cheap plastic when you tap on them. Possibly styrene... Codger posted this many years ago:
"Handle materials early on were mostly one of three materials; Metal (as on a lot of waldemar knives), jigged bone (also including antler and horn), and synthetics (mainly variations of celluloids). Each of these materials have negative traits which caused the knife manufacturers to try all types of alternatives.
The progress made in the 20th century in plastics development was key to the advancement of knife handle materials. Celluloid, somewhat corrosive and dimensionally unstable (and highly flammable), was replaced by more modern plastics which were in many ways just as nice in appearance, and much more stable over a long period of time.
Likewise, natural animal derived handle materials were by and large replaced with plastics formed to imitate the appearance and feel of bone, antler, and horn, as well as ivory. A revolution in plastics molding technology, co-developed with improved plastics, eliminated much handwork required in matching and fitting the natural materials.
Also, the advent of processes to anodize metals and plate plastics made it possible to own a gold or silver knife at a fraction of the cost of the genuine article.
Early plastic handles were a type of styrene. Styrene is more stable than celluloid, and cheaper than imported antler to produce and assemble, but still not both hard and completely dimensionally stable."
- V_P