Yes, the picture of the underside is helpful. The IC is definitely not an LM393 and it's not a common dual opamp either. It's a single opamp used as a comparator, probably a 741. The reference is not the 6V zener, it's the 3.9V. Not a very good choice since low-voltage zeners have high dynamic resistance and hence poor voltage characteristics.
Can't think of any other application off the top of my head (except the one about not allowing a product to run out of spec, like frequency drift). In fact, I'm not thinking clearly at all. It's almost 3:00 AM here now and I have a headache. Time to sign off. I'll look in again tomorrow. Bye.