Jay, are you thinking of Dardick, who made an odd pistol and lots of triangular plastic cartridges for both firearms application and oil-drilling applications, if I recall correctly? Some of the Dardick cartridges are self contained, while others are merely adapters with normal cartridges inserted into them.
I couldn’t be much help on earliest of plasticcartridges, but to get an accurate answer, I think you would need to define your parameters for “plastic ammunition.” Any cartridge with plastic used in its construction; plastic cases with heads of other materials; completely plastic cases; metallic cases with plastic projectiles; live ammunition (as opposed to, say, plastic dummy rounds such as the Germans used, with steel heads, or plastic blanks), etc.
Just a thought. I went through this once when I was going to do a cartridge show display of “plastic” cartridges. I gave up on it because I couldn’t decide what constituted “plastic ammunition.”