This is the logo for McCoy Pottery, a company begun by the McCoy family in 1910 in Roseville, Ohio. I went searching for it after my sister posted a picture of this green bowl on her blog yesterday as one of her "green" posts for the Shades of Inspiration challenge. I knew there was a long history behind the bowl and that it was a collectible. In fact, there's a whole society devoted to collecting McCoy pottery.
I also have one of the green No. 9 McCoy bowls, a gift from my mom and dad. I also have a yellow No. 11 bowl. In my searches for information about these bowls, I found that they are called McCoy Sanitary Stoneware Nested Mixing Bowls. Here's one for sale, priced at $95.
Here you can see the size difference between the No. 9 and the No. 11. I've used the No. 11 for mixing cakes many times--it's great for making an angel food cake since it's large enough to contain the fluffy batter.
After doing this research online, I've already put several books on reserve at the library about collecting McCoy pottery, pricing, etc. (Oh, one other thing I discovered is that the oldest of the McCoy pottery does not have the logo on it--that's why these bowls are unmarked except for the size indicated on the bottom of each bowl.
I wish the phrase "the real McCoy" came from this company and this product. That would make a great ending to my post. But alas, there are several other more likely origins for that expression. I'd bet that collectors of McCoy pottery think the phrase definitely fits their search for the genuine article made by the McCoy company.