This looks like a vinegar cruet and while "plastic" has been around since about 1843 (http://inventors.about.com/od/pstartinventions/a/plastics.htm
) the plastic that is probably used on this stopper is from the 1950's or later.
It very well could be that the plastic was put on the stopper at a later date but is highly unlikely.
I cannot tell much from the cruet as far as detail, pontil mark, etc. but it looks to be Bavarian in origin and from the style it looks to be from circa 1950.
Early plastics such as celluloid, vulcanite, casein, bakelite were hard plastics used in making jewelry, billiard balls, handles for small items, dresser sets, and many more items. They were not soft pliable plastics or vinyl.
Does you mother-in-law know any history on it or where she got it?