From The Apple Wiki

The Qualcomm MDM6600 baseband supplied with iOS 4.3, 4.3.1, 4.3.2, 4.3.3, 4.3.4 and 4.3.5 for the iPad 2 (iPad2,3). Please note that for the iPhone 4 (iPhone3,3) this was never released, as the iPhone 4 (iPhone3,3) didn't get the 4.3.x firmware, but instead the 4.2.5 series, which didn't include this baseband version.