01.51.00

This is an X-Gold 618 baseband, which can be found in the IPSWs for iOS 4.2.6 and 4.2.7. Its inclusion is peculiar, as the iPhone 4 (CDMA model) does not use the X-Gold 618; it uses the Qualcomm Gobi chipset. In addition, this baseband appears to predate 01.59.00, which was the initial shipping baseband for the iPhone 4 (GSM model).