Originally Posted by glowe

I will bet money that there was a thread in the distant past that pictured that exact style of target stocks complete with the plastic insert, but I am really bad at finding old threads on the Forum.

Bakelite was mostly out of favor by the end of WWII, replaced by thermoplastics. Thermosets were a very brittle form of plastics, which was a big complaint about Bakelite from the very start of production in 1909. By the 1930s, several companies were working with thermoplastics and Dow Chemical is credited with the first successful commercial launch of polystyrene, tradename Styron 666, marketed in 1938. Dow manufactured Polystyrene for the WWII effort and by the late 1940s, a high impact Polystyrene was developed, improving elastic properties of the product.

Many companies stopped using Bakelite in the mid-1940s as the demand for new and improved World War II related products took hold. By the end of the War, new technologies in the world of plastics had made Bakelite obsolete. My guess is that those inserts could have been made from polystyrene, partially because of what you noted, they did not break when stretched to fit the frame of your K frame.