William,
I'm still looking for information on your Menthodona Plaster. We have one in our Museum, but I'm still searching in our archives to see if I can find any information on it. Based on the name, it sounds as if its active ingredients were menthol and belladonna, two common medicated plaster ingredients. Both of them would have provided pain relief, the menthol likely providing heat/pain relief. In many cases when plasters had a combination of ingredients, the name was derived from combining the names of the two main medicinal ingredients -- for instance, Bellcapsic plasters (a popular 19th century plaster) contained belladonna and capsicum (the active ingredient in hot peppers) and would have provided pain relief and heat/pain relief too. As soon as I find out anything more about your Menthadona plaster, I will post it in the comments section of this post. I can say that it's one of our more rare historical products -- I haven't seen many of them at all around.
Regards,
Margaret