Gutta percha
Gutta Percha was an imitation? a synthetic? rubber like substance made from the sap of other than rubber trees. Henley's Formulas(1907) gives a process for making imitation gutta percha from the black extract of birch bark and india rubber.
The 1899 Parker catalog identifies the DHBP as the "rubber butt". This might be translated to the American version containing india rubber.
The fact that dhbp's and grip caps lighten indicates that lampblack was not used as a sunlight inhibitor, as it is in modern rubber products.
Best, Austin
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