In the Masoretic text of Tehillim 107 there are 7 pesukim which contain an inverted letter nun (similar to those in Sefer Bamidbar). In my standard edition Mikraot Gedolot none of the commentaries explain this. A Google search yielded only this podcast (which I have not been able to listen to yet). Therefore I would like to know what the purpose of these letters are.
The inverted nunim derive from the Greek scribal signs antisigma (ἀντίσιγμα) and diple (διπλῆ) which appear in Alexandrian papyri and in Qumran manuscripts. Here they are often used to indicate a verse or sentence in the wrong place.
Sifre to Num. 10:35 explains that there are dots on words in Numbers 10:35-6 to indicate that the verse is in its wrong place. The nunim here apparently serve the same purpose as the dots. Other rabbinic sources give similar explanations.
Nunim in Psalms 107:23-28 (TB Rosh Hashanah 17b, and commentaries for a rabbinic opinion) are harder to explain in this way, but it seems possible that they were misplaced verses too. Note that in the Leningrad codex (pdf), nunim precede verses 21, 26 and 30, and some Tiberian and Babylonian manuscripts do not mark nunim in Ps. 107 at all. The tradition regarding Ps. 107 is evidently weaker and less understood than for Num. 10:35-6.
There are markers in this chapter ... and they come to be expounded upon instead of “buts” and “onlys” to limit [the power of the verse] meaning that if they [the prisoners] cry out before the verdict has been promulgated, they are answered; after the verdict has been promulgated, they are not answered.