Ha is riffing off the symbolism of the Great Seal of the United States, which appears on the back of the US $1 bill, although the date on the base of the pyramid is 1776, the date of the signing of the Declaration of Independence and the official year of the founding of the U.S. Assuming it's not just the typo, it could be some Illuminati thing, or a reference to the National Treasure movies (which I haven't seen)--the symbols seem to be used as a literal seal on whatever's behind those doors.