I found a 1969 cent that sounds higher pitch than a 1965 cent. It appears slightly thinner and weighs less than the 1965 cent. The 1965 cent weighs 3 g and the 1969 cent weighs 2.6 g.
If the corrosive agent is strong enough to remove metal - acid and the like - it will remove the metal with near-perfect evenness, and the coin won't lose very much definition. It will just look like a slightly smaller version of the original. 0.4g is a relatively small weight loss in the grand acid scheme of things, so it's quite believable in this case. Acid coins are actually kind of cool. They can get pretty well-eaten before losing much detail.
Most likely corrosion as stated. But sometimes a Cent could be struck on a thinner Planchet. Here is 1970 example from my collection at 2.43g -
I'm pretty sure a lot of us has tested the acid type theory by throwing a cent into Coke. It does a number on a cent.
