The modem is an Ericsson F3507g P/N 43Y6477. It works in the x301 (obviously) and I've read that it works in the x20x and T61. It may work on others. This is the Lenovo part, so there won't be any BIOS compatibility issues on supported systems.
The going price on eBay is $99 to $119, so I'm asking $80 + actual shipping expense.
PM with an e-mail address if you need a photo, IMEI or whatever.
-darren




