[Llarry]

In the mid-1960s, when U.S. tactical aircraft first entered combat over North Vietnam, they had essentially no electronic support measures (ESM) or electronic countermeasures (ECM). The North Vietnamese likewise had a very unsophisticated air defense system.

As seems to be the case in all wars, that situation rapidly evolved. The USSR and China provided North Vietnam with radars, radar-control anti-aircraft artillery and ultimately surface-to-air-missiles. Without ESM, U.S. pilots would not even know that they were being targeted and without ECM would have no method of countering enemy radars.

The McDonnell-Douglas A-4 Skyhawk was the principle light attack aircraft of the U.S. carriers and of the Marine Corps squadrons stationed in northern South Vietnam. The A-4 desperately needed upgrades to minimize vulnerability to enemy air defenses. At the same time, the A-4 Skyhawk (also known as the "Scooter" due to its compact size) did not have much room to add electronic components. The solution came in the form of a hump in the dorsal area behind the cockpit. Late model A-4s: A-4Es and A-4Fs, were retrofitted with this hump along with the associated electronic black boxes. The final A-4 variant, the Marines' A-4M, came straight from the factory with the hump. The vulnerability of the A-4 was reduced and all was good.

The hump became a way to date a photograph of A-4 variants -- hump = 1960s-on. The Reserve force got in on the action, too, as older A-4Cs were upgraded with the hump as A-4Ls. The Marines even fitted some two seaters as forward air controller aircraft as the OA-4M and these had the hump as well.

On the other hand, as @smokinjoe64 can attest, A-4s used for adversary training usually had the hump removed; for adversary training, lightweight and low drag is good.

Photos:
1 to 3: A-4E, A-4F and A-4M
4: OA-4M
5: A-4L as retrofitted
6: Adversary A-4F "Super Fox" with hump removed