Except that it isn't a glottal stop. It's basically a constricting of the throat itself, it doesn't come from the voice box. An apostraphe is perfect for glottal stops. Usually an ayin in Arabic is actually transliterated with a backwards apostraphe, but that isn't available via this font/typing system. The 3 is also used sometimes, especially in non-academically typed transliterations, and has the advantage of looking like a stand-alone ayin, just shifted up a bit. I'm not sure why that matters- but I guess as a mnemonic for something rather unusual...