Apple has posted a KBase document on iPod (with Dock Connector): How to Install Dock Cover M9130G/A:
“If you buy an iPod model that includes a dock, the dock cover is already installed.
.If you buy an iPod that does not include a dock, or if you buy an extra dock, you must install the cover that fits your iPod model. The 10 GB and 15 GB models are slightly thinner than the 30 GB model, and they use a dock cover that has a thinner opening.
This cover is labeled “10/15GB Dock cover’. The cover for the 30 GB model has a slightly deeper opening.
It is labeled ‘30GB Dock cover’.”
