The P-38 had a mass balanced elevator with external counterweights from day 1. Google images of P-38 prototype and you'll see them.
Please refer to this article, second paragraph below is a cut & paste:
https://en.wikipedia.org/wiki/Lockheed_P-38_Lightning
Other info from various printed sources.
Lockheed’s P-38 was the first fighter to fly faster than 400 mph. It was already in production in the fall of 1941 when some alarming problems started showing up in test flights: in high speed dives the plane would shake violently, the controls would seem to lock up, and the nose would tuck under, sending the plane into an ever-steeper dive. On November 4, 1941, test pilot Ralph Virden was killed when he could not pull out of one of these dives.
In 1941 flutter was a familiar engineering problem related to a too-flexible tail, but the P-38's
empennage was completely skinned in aluminum
[Note 2] rather than fabric and was quite rigid. At no time did the P-38 suffer from true flutter.
[38] To prove a point, one elevator and its vertical stabilizers were skinned with metal 63% thicker than standard, but the increase in rigidity made no difference in vibration. Army
Lieutenant Colonel Kenneth B. Wolfe (head of Army Production Engineering) asked Lockheed to try external mass balances above and below the elevator, though the P-38 already had large mass balances elegantly placed within each vertical stabilizer. Various configurations of external mass balances were equipped, and dangerously steep test flights were flown to document their performance. Explaining to Wolfe in Report No. 2414, Kelly Johnson wrote "the violence of the vibration was unchanged and the diving tendency was naturally the same for all conditions."
[39] The external mass balances did not help at all. Nonetheless, at Wolfe's insistence, the additional external balances were a feature of every P-38 built from then on.
[40]
All the experts consulted were baffled until John Stack diagnosed the problem: the compressibility burble was causing the wing to lose lift; that altered the angle of the air leaving its trailing edge and striking the tail, in turn generating increased lift on the tail which pitched the tail up—and the nose down.
The '38 was first plane to suffer from compressability because it was the first one to achieve the speed required to hit compressability. The "dive recovery flaps" installed on the 'J-25 and L models didn't fix compressability, but made it possible to manage it. The F4U and P-47 also ran into compressability issues, with at least the '47 also being equipped with dive recovery flaps.
This situation was aggravated by the early problem of the nacelle slipstream going supersonic and essentially locking the rear elevators. Deadly in a dive when exceeding a certain speed. Took them several planes to figure out what was going on. A small flap was added to break up the stream. "Fix-kit"s were sent out to retro fit early models. The aircraft carrying them to Europe was shot down and they did not arrive. The P-38's were not allowed to follow the enemy planes in a high speed dive until the production airplanes were equipped with the dive recovery flaps.
The elevator balance weights were originally internal but had to have mounted external counter balance weights to satisfy misconceptions of the Air Force brass.
Yep. External ones were redundant, it retained the internal ones.