However, at the time there was a misunderstanding concerning Rav Moshe’s personal conviction and some people mistakenly supposed that he was even in support of an eruv; hence Rav Moshe’s two teshuvos clarifying his opinion (ibid., 4:87-88). Even when clarifying his position, which he considered to be “l’dinah,” Rav Moshe did not want to issue a p’sak since, as he wrote, his chiddush was not mentioned in the Achronim, and moreover the Aruch HaShulchan would not agree with him. It is important to note that both these teshuvos (ibid.) were written to the same people and the second one was just a clarification of the first and was not written as a p’sak to prohibit the eruv. In the first teshuvah (ibid., 4:87) Rav Moshe explained his chiddush that in order for there to be shishim ribo traversing the streets over a twelve mil by twelve mil area there would need to be about 3,000,000 people living in the area. However after Rav Moshe was told that there were fewer than 3,000,000 people living in Brooklyn, he expounded on his chiddush in the second teshuvah and added that he would not be in favor of an eruv in a big city since some may think that there was shishim ribo traversing the streets (ibid., 4:88; see Three Million: More or Less?). In both instances, “l’dinah” refers to Rav Moshe’s personal opinion.
Concerning the Boro Park eruv, a similar sequence of events occurred when Hagaon Rav Menashe Klein shlita asked Rav Moshe to issue a p’sak whether he could erect an eruv. Rav Moshe answered that he could do as he saw fit (Oim Ani Chomah, p. 266). Just as in Flatbush, this was misunderstood to mean that Rav Moshe would support an eruv. Therefore, Rav Moshe felt a need to clarify his personal opinion on this matter (Igros Moshe, O.C. 5:28:20). In both Boro Park and Flatbush, Rav Moshe’s teshuvos were written in reaction to the public’s misunderstanding of his personal opinion regarding eruvin and not as a p’sak. [Concerning Rav Moshe’s signature on the 1979 kol korei which prohibited the Flatbush eruv, see The 1979 Flatbush Kol Korei Exposed.]
On the other hand, when Rav Tzvi Eisenstadt zt”l asked the rabbanim of Manhattan for their opinions ― as opposed to their p’sak ― concerning erecting an eruv, Rav Moshe answered that in his personal opinion they should not establish an eruv (see the beginning of Igros Moshe, O.C. 1:138 which is the beginning of the Manhattan teshuvah 1:139). When Rav Moshe was asked if the rabbanim of Manhattan have a right to erect an eruv he answered that they could do as they saw fit (ibid., 4:89 and HaPardes, 33rd year, vol. 9). This is exactly what Rav Moshe told Rav Menachem M. Kasher zt”l, “even though I will not join you, you should go finish what you started [and establish the eruv]” (Divrei Menachem, O.C. vol. 2, introduction). Only when Rav Aharon Kotler zt”l and other Roshei Yeshivos made a takanah not to erect an eruv in Manhattan did Rav Moshe join them and prohibit the eruv as well (Igros Moshe, O.C. 4:86 and Addendum to O.C. 4:89) ― this takanah was only about Manhattan and not Brooklyn; see The 1979 Flatbush Kol Korei Exposed. Even when Rav Moshe signed on the 1962 takanah with Rav Aharon Kotler we see that he was not at ease with the language which stated that, “those who rely on the eruv in Manhattan are considered a mechalel Shabbos,” since after he quoted this takanah in his teshuvah, he omitted this last line (ibid., Addendum to O.C. 4:89).
Given that Rav Moshe, despite his personal convictions, never issued a p’sak for others against erecting eruvin we can possibly understand why Rav Moshe stated (ibid., 3:94, 5:19, 5:24:10) that we rely on the heter of shishim ribo in all cities ― even Brooklyn and Manhattan ― since he did not want to employ his chiddush as a p’sak (see also Rav Moshe zt"l and the Walking Stick).