Looking closely at the several neutral busses here, it appears that the burn/overheat marks are on the conductors which are attached to the top-most buss (the one closest to the breakers). I wonder if a poor connection exists where the two phillips screws connect this buss to the rest of the neutral assembly, causing this entire buss to heat up?
#104988 - 03/20/0409:01 AMRe: Recycled Image from the past!
Chances are, the main overcurrent protection (breaker) is at the source for this panel.. In the eastern side of the USA, it seems to be popular, if not universal to have the power co.'s meter outside with a main disconnect.. This in-turn feeds to a distribution panel elsewhere.. (usually a garage or basement, which is what you see here... Protection from overloads for this type of panel would be at the source it was powered from.. (Main disconnect, branch circuit feed from another panel, etc.)