E4421B-1E5 High-stability time-base
E4421B-1E6 High performance pulse modulation
E4421B-1EM Moves all front panel connectors to rear panel
E4421B-1EN Additional material for 1E6 and UNN
E4421B-6NB Additional material for 1E6 and UNB
E4421B-UK6 Commercial calibrations certificate w/ test data
E4421B-UNB High output power with mechanical attenuator
E4421B-UNM Standard front and rear panel connector configuration
E4421B-UNN Standard output power with electronic attenuator
E4421B-UNQ Standard stability time-base