[Sashimister]

I might add this so there would be no misunderstanding about プラ.

We actually never shorten プラスチック to プラ in either spoken or written Japanese. We always use the former. In these recycling symbols, however, プラ is used probably because プラスチック is too long to blend into a simple design.

If you use プラ in any kind of conversation, we will have absolutely no idea of what you are referring to.