Raimey, I think buttplates were mistakenly called Bakelite by many people over the years. Certainly lots of buttplates resemble that product , but of course, some come decades before Bakelite. I have also heard guttapercha was used. There are quite a number of patents relating to this product and I think the product was used in Colt pistol cases maybe as early as the 1860s.
Our friend in Belgium has asked a friend of his, who is an antique expert, about our question. Now that we have the Belgium lead, it may explain the lack of information on these products from the U.S.
