Nortek has several families of instruments: pre-Midlife, Midlife, and AD2CP. These terms generally refer to the electronics platform that is used in the instruments.
Pre-Midlife and Midlife instruments include the AWAC, Aquadopp, Aquadopp Profiler and Vector. Pre-Midlife instruments have longer electronics boards, so can only fit one 50 Wh battery inside a long housing. Midlife instruments have shorter boards, so the long housing can fit two 50 Wh batteries or one 100 Wh battery. Pre-Midlife firmware can be identified with the format 1.xx, while Midlife firmware is of the format 3.xx. Both Pre-Midlife and Midlife instruments are narrowband systems.
AD2CP instruments include the Signature and DVL families. They are broadband instruments that use a board stack rather than one main electronics board. Firmware versions are in the format xxxx_xx.