I still have this from Coin Magazine. They use to give out these and other types of gift to renewal your subscription.
Funny.. Why would they have the S in quotations? They could of simply stated.. The S mint mark distinguishes coins
The correct punctuation woud be 'S' using single tick quotes. Double tick quotes are for indicating a quoted element of speech, the single quote mark is to demarcate an element in a sentence that is not a grammatical part of the sentence but is being referred to in the sentence. In this case the letter S is not a part of speech, not a noun, verb, etc. but needs to be included.
Those were the good old days when subscription people send you good stuff rather these nothing and cost you more to subscribe.
This is like a Magellan GPS I had. A local road is South Park Road, abbreviated, of course S. Park Road but the Magellan voice pronounces it as Spark Road.