The earliest source of this story is Blunders in International Business by David A. Ricks (first edition 1974). He basically repeats the story you heard. The book has few footnotes, but this story does have a footnote, which goes to his personal correspondence with the Parker Pen Company.
Although many people have cited the “pregnancy” problem
Macbeth dates back to around 1606.
Tongue of Dog
While the most common name for the plant seems to have been hounds tongue, there are a few instances of it going by another name: Dogs tongue.
I found “Dogs toong” in the The Herball Or Generall Historie of Plantes (1597), page 337.
Here’s another quote, from the ME translation of Guy de Chauliac's Grande ...