Advantage and K9 advantix are two different companies, I believe.
There are instructions for identifying the fakes at the EPA’s website.
STrange our Vet had told us that Advantage was for Cats and Advantix was for Dogs.
They are the same company, Bayer. One, Advantage, does just fleas, Advantix does fleas and ticks, and Advantage Multi does fleas, heartworm and some intestinal parasites.