What you are seeing on the lower loop of the mint mark is from a damaged punch. If you look at the listings on coppercoins for the 1955-S RPMs you will see it on them and other normal coins from that year. At least until they replaced the punch, which was doubtful because the mint closed that year. Thus no more need for the mint mark punch. (At least what they thought, as the mint was reopened in 1968)
Tip of the day
Dies go through the same events over and over. So markers come as a secondary. The first thing is to find the variety, then the markers. The markers mean nothing on a normal coin.