1,999 letters, with 72% in the 1950s — the campaign decade. The rule itself is a release: in these last generations, it is permitted and a mitzvah to reveal the Torah's inner dimension. Through this study, specifically, "they will go forth from exile with mercy." But the restraint never falls on access — only on method: don't over-engineer the study; earlier generations learned simply and knew more. It must become inner work (avodah), not mere intellectual grasp.
For most of Jewish history the inner Torah was guarded — revealed, he notes, only to Rabbi Shimon bar Yochai and his circle. His standing claim reverses that: in the last generations it is mutar u-mitzvah to reveal it, and through this study specifically "we go out of exile with mercy." The whole thread is a campaign to bring the sealed wisdom into the open — as תורת החסידות, Kabbalah made livable.
The recurring claims about the inner Torah, by how often each surfaces across the cluster.
The distinction that runs under all of it: not raw Kabbalah but תורת החסידות — the same inner light, rendered in terms the mind can hold. He sends a beginner not to the Arizal but to Tanya and Derech Mitzvosecha.
He pushes the inner Torah outward as far as it will go. So his restraint is easy to miss — it never falls on access. It falls on method: don't engineer the study into a system, and don't let it end at the intellect. The point is the maor, the light-source — and the sign that you've reached it is that the learning turns into avodah. Here, verbatim.