Lucky you for being able to fish it out. I figured the odds were you'd have to pull the rad and shake it upside down just to see it. In that case I would have suggested just leaving it and getting a new grommet.

For proper installation, mount the thermostat in the grommet then push that into the opening. It's supposed to be a dry fit, but using water as a lubricant makes life easier, just don't use anything petroleum based. What with plastic side tanks, do not be tempted to hammer it in, rather press it in. I now use a carpenter's F-clamp with a block of wood behind the rad to prevent damage and a scrap of wood or metal bar between the thermostat terminals to prevent damage there. When you're done, run one or two zip ties through the core and around the tank to secure it from ever blowing out -happened to me once. Use black UV-resistant zip ties and the fattest ones that will go through the core to maximize longevity.

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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now