There was a famous crash in the ‘55 Le Mans that featured a magnesium alloy Mercedes, although the details are not as describe.
Dutch Gran Prix in the early ‘70s had a spectacular magnesium burn crash.
I guess it could have been 55 but I still think in the 30’s.
They did show it all. The film was B&W but it still could have been B&W in 55 too. I do remember quite clearly the first guy with the extinguisher going to great effort and heroism. He did everything he possibly could and subjected himself to real danger. He finally just ran out of extinguisher and went away weeping.