[VadeRetro]

IHS = Isis, Horus, and Seth, the gods of Egypt!

Yes! It's obvious now that Jack Chick points it out. (Of course, he seems to have forgotten Osiris, Amon Re, Bastet, Nut, Sobek, Aten, Tehuti, and a few dozen others.)

[Doctor Stochastic]

It's too tempting:

The great god Ra whose shrine once covered acres
Is filler now for crossword-puzzle makers.
- Keith Preston (1884-1927)