The 55 actually looks like it might be a tiny lamination. Laminations themselves aren't that unusual, but this is a kind of neat one. The S on the 75S just has some die chips inside the S. They're pretty common.
Can you take a picture of the last A on America in the first Wheat cent reverse you showed? I think I see doubling.
I disagree about the blithe statement concerning the filled S on the 1975 penny. I do not know what other error could have caused, perhaps a RP mm, but why are die chips floating inside the press during minting? There are many coins with mintmarks that are flawed. We need a good resource to research the possible cause. Frank Spadones' book, "Major Oddity and Varieties of U.S. Coins, " [© 1947], is such a book, cataloging every error he could find in u.s. coinage up thru 1970, the revision I have. I implore all serious collectors to find this book and marvel at its consummate completeness.
On the 55 those are die chips. On the 75 the S punch was deteriorating. Spadone's book also included a lot of minor errors and frankly just plain damaged coins and even counterfeits.
jonny.. I have a thread concerning Frank Spadone and his reference books. I own one of these books and many of the senior and error experts considered most of his information inaccurate and false. So not the best source for good reliable references. Also.. A die chip is actually a chip on the die striking a blank planchet.. not chips of metal floating around. Here is my thread . Read all the posts. https://www.cointalk.com/threads/1981-book-on-variety-and-oddity-us-coins-my-first-source.261195/ One statement was - "Spadone's book is dreadful. Full of alterations and counterfeits. The terminology is idiosyncratic and obsolete. Many a novice collector has been led astray by this wretched excuse for an error reference."
I can see the doubling, but it looks shelf-like to me, indicating MD. Also, no DDRs for business strike coins are currently known for 1955. (There are several for proofs.)
