The following is from page 711 of HaTekufah HaGedolah. I'll translate the part after the letter.
On the outside of the letter is written:
למו"צ דק"ק יצ"ו
דברו אל י'ש'ר'א'ל'
Inside the letter itself is written:
"יום ג' פ' לך לך. מאלקא רבא קדישא, דשכין שמיה בקרתא קדישא, שלמא רבא ונפישא, לעמיה קדישא ופרישא, ע"י הדא איגרתא קדישא, דהיא זעירא ורבי' פרישא.
מיבע שטמט מטדמל
אביו שבשאמ נאי בתערמ ותותוצ מארה ללא וללא וימצב ותדוא וי"ח לתקע"בג וילי הכד אוי אמת ואמכם ציון
והאות על זה
שבד' מספר כ"ו ינוח שם רמ"ח בן מ"ב עם אבותיו שם ב"ן ושם ס"ג
שמי א' ש"א.
According to the tradition it is told in the introduction to the letter that was brought to print (Warsaw 5696) that the GR"A left after him a closed letter with an attached instruction not to open it until 100 years had passed since his death.
When the time came, the letter was opened in the "קלויז" of the GR"A, but no one knew how to decode its secrets. The wise men of the "קלויז" sent a copy of the letter to all the important communities which had mekubalim, so that they would try to decode it. When the letter reached Yerushalayim it was printed in 5658 in the periodical "Torah MiTzion" (3rd year, number 3).
In 5662 it was brought to print by Rav Pesach Cohen Pinfer in his book "The Tradition of the Torah and the Prophets", and on the sides was printed a letter of Rav Shmuel Chaim Luria to the publisher, saying that "the letter of the GR"A is in my hands", and that he got it from the Rada"l. He added that the words of the Rada"l that "there is no man in the world that can write an explanation for this, but in the future Eliyahu will explain it". Rav Pesach was requested himself to try to decode the letter.
The letter was printed again in the introduction to a book "Divrei Eliyahu" of Rav Avraham Dovid Bloch, from the students of "קלויז הגר"א", and also printed and distributed in many forms by Rav Pinchas Arkin from Briansk. Rav Pinchas would say that he knew the secret of the letter, but he said it was forbidden to him to reveal it in Chutz La'Aretz, and therefore he asked that he be assisted to go up to Yerushalayim, there he can reveal the secret. In the days of the gathering of Rabbanim that took place in 5695 in Warsaw, Rav Pinchas arrived to the offices of "Agudas Yisrael" and gave a copy of the letter to Rav Zusha Friedman. But he didn't reveal to him its secret, and he passed away in holiness and purity on the steps of the mikveh in Otbotsk, and the secret, he took with him.
In the year 5696 the letter was brought to press by Rav Y. Dimdus, and he also tried in his powers to achieve the meaning of the letter, saying that the GR"A hinted at the secrets of the Geulah.
On the side of that printing, it was written that in the near future there will be a detailed explanation of the letter, with the agreement of the great mekubalim, at their head Rav Shimon Tzvi Horovitz of Yerushalayim, but nothing is known of this printing.