So...this is Gutta Percha? Since it predates the Bakelite invention date (& it's clearly not horn) it likely is just that. Thankyou yet again Dr. Drew. I guess chemically this material is a "polyturpene", specifically
trans-1,4-polyisoprene. Neat stuff! This 1890 Quality 2 buttplate and grip cap are almost an olive-brown.