Celluloid (cellulose nitrate) was used by USPC until they stopped making chips (late 1940s I believe). I also believe, although I could be wrong, that Burt used celluloid early on (into the 1950s) then switched to plastic.

I believe celluloid was discontinued in chip making for 2 reasons: 1) the extreme fire danger presented. Saying celluloid is fammable is like saying Warren Buffett is rich. Celluloid burns fast and hot and produces thick, toxic smoke. 2) probably more important (back then anyway) is that celluloid had to be imported in sheets (I think from Italy) and was quite costly.

I credit (or blame) the above information to Jim Blanchard.