Shack, one of the smart guys on our course, ex-ranker, had PLASTIC collars. They looked the real deal even down to the pattern. The give away was the sheen stayed on whereas it wore off on the starched collars.
I wore plastic collars from the time I entered man's service in 1961 until I finally entered civvy street in 1973. I am sure that 99 percent of the chaps that I encountered also wore plastic collars.