According to you, what is the eldest Canadian $1 coin? That of 1987? No, though it officially replaced the $1 note, withdrawn at the beginning of 1989. Then it should be that of 1935? You're partially right. As a matter of fact, it's the first one that was issued. The eldest one is the $1 of 1911.
This is one of the most obscure stories ever known about a coin. The coin was even never issued. Back in 1910, the Canadian government amended the Dominion of Canada Currency Act, stating that a silver dollar of .925, 36 mm and weighing 360 grains (23.25 g or .82 ounces) should be issued in 1911. Of course, the coin had the same designs as those made out in cents back then. But the project was never to be completely fulfilled. Even the records don't have a lot of information about that..
Thus, for about 50 years, the coin was considered as nonexistent. In 1960, the story made a turning point : B.A. Seaby, a British dealer, bought a specimen from an ordinary person. A few years later, he discovered a second specimen. Then, in 1977, a 3rd specimen was discovered. Of all the three known specimens, it's the most mysterious one. Why ? Because that one was struck in lead, not in silver. It was discovered at the Ministry of Finances former buildings. Very little is known about that lead specimen, even as of 2010. However, you can see one of the two silver specimens at the Bank of Canada Currency Museum, the other one being housed in a private collection. Today, it's still the rarest and most valuable Canadian coin.