You see if it opens from a cold state and opens quite quickly. I guess, since I've never tried to take one apart, that it's pressed in somewhere under the isc ports. If you can determine where it is, and if that air path will hold water with the thermostatic fidle closed, pour warmed up water on it, raising the temp until it opens and lets the water go thru. I have no idea of the opening temp, and I've never seen the spec for it anywhere. I know it opens fast, so the open temp must be low, but how low, I don't know.

I'd try one of the online parts houses if the stealer doesn't have one. Or there's a bunch in the jy's. Not much to go wrong with one except the capsule and the shaft bushings.