I Have seen it before but not positive about this information. I think it is from a hair oil or lotion bottle. there was a little bit of cork on nub of stopper. When you dig something like that. The cork is usually gone.
Nice Stopper, I found these only inches apart - this stopper also has 2 N and 2 H letters...the bottle is embossed Newbro's Herpicide (not sure if the spelling is with a B or a P.)
Hope this Helps....BillT.