How to identify iPad model based solely on Serial Number?
This Apple page checks support, warranty, and AppleCare availability for Apple products.

Entering an Apple device serial number, however, will also display that device's model and type. I just tried it with the iMac on which I'm typing this answer, and it returned the correct information. Should work as well on your mystery iPad.

EDIT: `This more specific Apple page identifies the iPad model from its serial number. I don't have an iPad so I can't check if it provides the detail you want.