Strange 50 pfennig 1921

I was putting the rest of my German coins into my album, when I noticed this coin:



It looks like a 50 pfennig 1921 J, just a lot more worn than the "normal" one on the right. It is, however, much thinner and lighter than the normal one.



It is less than 1mm thick, while the normal is 1.5mm like it is supposed to be. The normal one weighs in at 1.64g (almost exactly what it should, according to its Numista page), while the thin one weighs only precisely 1.00g.

Any explanation for this?

In the date on the black one it looks like the third number is an upside down turned 3?

Indeed striking. Judging from its appearance I would conclude that it is substantially corroded rather than worn. Perhaps it has been in the ground for a long time and lost much of its weight from corrosion?

I've read somewhere that silver coins which spend a long time in the ground become lighter because the copper of the alloy 'washes out'. I've no idea how aluminium responds to this kind of exposure to the elements.

Not an upside down 3, just a normal 2 but not so clearly anymore because of the corrosion:



And ArnoV, I do believe corrosion effects the weight but I don't believe loosing so much thickness with still visible obverse and reverse. So there must be another reason for this different thickness and weight.

Quite strange - look at how the rim has been worn right down to maybe half its normal width. Is there a significant difference in the diameter of the two coins?

The worn coin is approximately 1mm smaller than the non-worn one.

Just speculating here but it's the time of the hyperinflation and to save on costs maybe they used thinner planchetts (then there should be more of the 21 and 22 years around that are as thin as your speciment). Because of this there could be a loss of detail at least in the first trials with the new size or nobody cared because the small change was getting more userless as time went by.
The heavy wear aluminium is a softmetal and maybe it circulated a bit more than your usual coin.
I don't know if those coins are out of an alloy or pure but if its 100% Al it's quite unlikely even if it was burried in the ground for decades ... but if it was in an acidic environment there should be deep little holes if it was in an alkaline one shallow wells.