The grip scales look like they could be cast/moulded in Gutta-percha, a tree resin frequently used for such and made impressing designs like that possible. I doubt you could do that in horn. It was used extensively from the 1840's thru the early 20th Century. It is still used for human dentistry, as an inert and safe filler for the void left by a root canal procedure.
See
Antique Gutta Percha Handles