1973 Lincoln Cent Spiral Error?

Thank you Dearborn. Ill look in to it

I have some coins from same period with same kind of staining. We do not find not trace of a major acids, but in the end the SEM-EDX show us that was come from black coffee. By mean the coffee it is acidic and in fact some how has those traces on this coin, which can have unexplorable possibilities. The fact it is the acid from coffee attack the zinc not the copper, and for this reason on this coin we find the specific color of the zinc acidic attack.

The reason the stain doesn't appear on the tops of the letters is because they are the highest point on the reverse of the coin and thus will wear the most. Notice that all the letters have an even color? They've all worn during circulation more or less equally and the stain was on them and has now been worn off.

Also, the stain doesn't skip the devices. Notice that it runs over the top of the monument and also up the stairs? Those areas wear less and show the stain is old and hasn't worn completely off the coin.