Was this chamber mentioned above?
Radak citing Targum Yonasan: No - the only chambers mentioned that faced south were of Leviyim, and this one was of Kohanim.
Vilna Gaon: It is a room of "Lishchos Sharim" (44). This is why it says here "Lishkah" (singular).
Malbim: Yes - this is "Lishchos Sharim." Refer to 40:44:3:2.
Who are these Kohanim?
Rashi: They are Mesharsei (serve) the Bayis. They are Leviyim, who sing 1 .
Vilna Gaon: They are those who sinned with idolatry. They do not serve [like Kohanim]; they do Shechitah, and guard the Bayis.
Malbim: In Perek 44, it says that the only Kohanim who will serve are Bnei Tzadok, who did not stray from Hashem. The others stayed with idolatry. They will not serve on the Mizbe'ach. Rather, they will guard "Shomerei Mishmeres ha'Mizbe'ach."
Radak disagrees; he said that this cannot be of the Leviyim. (PF)
