Ok everyone I have studied this bill for a while now and I am certain that the extra letters MX on the left-hand side are 100℅ under the security feature of the tiny 100's, which means it was printed before the security feature, right?
That looks like a partial teller stamp from a bank, the yellow 100s are showing through it probably because the ink didn't adhere to them the same way it did to the paper around them.
And it's clearly printed on top of the security features. It's simply that the alcohol-based stamp-pad ink isn't sticking to the intaglio printed features.
The ink in the stamp doesn’t adhere to the printed surface of the note and “appears” to be under it. I assure you it isn’t. Questions about stampings and annotations on banknotes are the leading questions from new collectors. It’s a Chopmark. Read all about them here - https://www.coinbooks.org/esylum_v17n07a21.html