1988 Lincoln Penny No Mint Mark.

It's missing the t in trust and looks like the words are mirrored? But not cool though.

The ridge ring--indicative of die deterioration--contributed to the distortion of the lettering in TRUST. Not that uncommon with ridge rings. Nice pics!

For most years of the cents production...there was no "P" MM for those produced in Philadelphia. I think it started in 2017 adding a "P".

KK

While the answers are all correct, what you are looking to find with the 1988 cent is the rare reverse of the 89. In 1989 the reverse die had some slight changes in the designer's initials that was discovered. While there has been much speculation to the numbers of these transition coins, my research tells me that when dies are changed for a new coin (year) both obverse and reverse are placed in the stamping machines at the same time. With the 1988, we found coins that still had the 1988 obverse mated with the 1989 reverse. The reverse die is on the bottom of the stamping machine, which produces about 12 coins each second. Philadelphia specimens number 9 die samples while the Denver number is 11 I believe. There have been a few of the Philadelphia specimens found, but those Denver are extremely rare. Since the stamping machines must be constantly producing 24 hours a day, it is speculated the coins produced with the 89 reverse were limited to how many seconds the machine ran from the time of installing the lower die to installing the upper. Since dies are keyed to only go into the machines on one side and a single direction. These numbers are probably pretty low, and certainly not even close to a full die run, since it was a complete year coin change over, with both sides being replaced as a unit at the same time.

That would be a very inclusive explanation of why there is no MM on this cent.
but your coin is suffering from a Ridge Ring.