As you can see, this letter, dated 24 September 1781, says that Mendelssohn promises to send him a copy of
"An allegorical drama. The author lived in Amsterdam about 50 years ago, and was a great genius in many respects. He was unable to develop his talents due to the jealousy of some rabbis, and was treated poorly. He retreated into solitude and died before his time. His Kabbalistic manuscripts are today studied intensely in Poland. He evidently wrote some new Psalms, which I have not had an opportunity to see."
The "allegorical drama" is obviously לישרים תהלה, which had been printed in a very limited run Amsterdam in 1743, and then reprinted by Mendelssohn's colleague Solomon Dubno in 1780. Dubno had spent several years in Amsterdam before coming to Berlin, and there he became acquainted with the writings of Ramhal through his students, such as David Franco Mendes. He really loved this particular play - which was written in honor of someone's wedding. Dubno reprinted it because it was so rare. He writes that לא הדפיס ממנו רק חמישים ספרים ולא הובאו כי אם אל אוצרות הגבירים הספרדים שבאמשטרדם יצ"ו.
Funny how things work - the original 1743 edition, of which only 50 copies were printed, is available online, while the 1780 reprint is not.
The Psalms of which Mendelssohn writes are a series of 150 Psalms that Ramchal wrote when he was about 20 years old. They caused him no small amount of trouble, as it was seen as some sort of attempt to supplant the actual book of Psalms. To my knowledge it was never printed and may not have even survived. However, two were printed in Bikkure Ha-'ittim in 1825 and 1826. Here they are:
The reference to the Kabbalistic manuscripts being studied intently in Poland is because Ramhal's Vilna-born Paduan medical student pupil Yekusiel Gordon was responsible for disseminating many of his manuscripts in Brisk and elsewhere in his native Lithuania.