Actually, I am older then dirt. Boot camp 1966 and ET A school at Great Lakes, NPS at Mare Island, Vallejo CA, then on to West Milton for prototype (D1G for me), then the fleet (Bainbridge just finishing up her first refueling at Mare Island).
Given there were only 4 nuke surface ships at the time, your choice was the Truxtun, Bainbridge, Big E, or the Long Beach. Duty rotation was two years sea then two years shore. You guys seem to have it harder now then then.
Oh, back then there was only ETC (communications) and ETR (Radar, TACAN,...). ET's were also trained in electronic warfare equipment and encryption equipment.
The ETN designation seems like a good idea as long as there is a large enough pool to allow advancement. While the ETC and ETR tests had nothing to do with nuclear power, there was a vast pool of people and openings for advancement fleet wide. Couple that with the extra points you got for being a nuke, rapid promotion was the norm, but it did create top heavy RC divisions. At the time, there was no extra money for nuclear NEC's, so they compensated with promotions.