Thermostat "o" ring?

[Feelstranger]

Folks, just pulled my thermostat and found that the sealing gasket was a square edged ring - not an o ring.  However, all the usual sources indicate it is an o ring.  
The way the housing fits to the gasket seems like it is designed for the original flat sided gasket, not an o ring.
I suppose it is *possible* that what I pulled out *was* an oring when it went in, and has reformed it self under stress.
What's the deal?
Thanks!
Nat