I got my Classic K-15 brewer (my first Keurig)
 today for Christmas (S/N 31.3100.0338650), and there's no way the "My K-Cup" that was purchased with it will fit in the K-Cup holder (yes, I removed the adapter piece per the sticker on the"My K-Cup" so it's only a gray plastic cup with fine mesh "windows" and a black plastic cap.

The "My K-Cup" box says it fits ALL Keurig brewers, but it sure doesn't fit mine.

The Keurig brewer works just fine, and I recommend it, but I can't recommend the "My K-Cup" device.