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Are you sure the Masons put the year on it, and it wasn't originally a shul or some other Jewish institution like a YM/WHA? There's a movie theater near NYU (on 2nd Avenue around 10th Street) that has a Hebrew year on the cornerstone, I've always assumed it used to be a shul.
Talia: It was never a shul, as far as I know. It was a Yiddish theater back in the day (1920s-30s), one of maybe a dozen or more that lined 2nd avenue between Houston & 14th street.