Is it just me or does the top 1955 S mint mark look like it's upside down? The other pic is for comparison
Check this out. In 1946 there were three different mintmark styles used by the San Francisco mint. These styles are the Sans Serif, Serif, and Ball Serif. The Ball Serif is scarcer than the other two styles. This die variety is a 1946 S Lincoln Cent with an inverted mintmark. The mintmark was actually struck upside down. This is the only known inverted mintmark in the Lincoln series, and it can only be seen on the Ball Serif style 1946 S cents. This variety is extremely rare. It is recognized by Coneca and Wexler. ANACS will certify this variety.
After checking RPMs on doubleddie.com, I don't see any for the '55s. That doesn't mean that there can't be one out there somewhere though, if you do think you have one you can submit it here.
I can't show a photo of my coin.Because I don't have it in hand, plus my scope is not working.But if you search 1955 triple S on this site you will find my post.Coneca.org also has photo's of the triple S.Maybe a member will kind enough to post you a link to it.
Of course it's possible for there to be another inverted mint mark. But think about it. The S is hand punched (upside down into one of the dies). That means there are hundreds of thousands of these inverted mint marks in circulation, and in 60 years no one has discovered one. It happens, but it is unlikely.
I don't believe any one was looking for them till the CPG started to list them. Do you have any idea when they listed them?