The brass bridge broke right in the middle where it holds the piston. No sign of stretched metal. It looks like the break developed over time as the broken edges are not uniform in color. It looks like the edges of a fractured metal casting. Does not appear to be any missing metal, possibly a few grains. Dodged a bullet when I took the thermostat out. The piston has a small brass fitting held in place by the bridge. The piston was pushed up past the broken bridge and this fitting could have easily come off. It may have been prevented from falling off by underside of the thermostat housing. In hind sight, you should be careful when taking off a suspected broken thermostat because pieces may fall back in. Good thought, Tunk , I will drain and flush. I cheated and just replaced the lost coolant (about a quart) with water. I have some new MBZ coolant but wanted to get some citric acid and do a complete flush.
Cooling system appears to be fine. I'm keeping an eye on the fan clutch. Already bought a new one, $130! yikes!
In hind sight, I think the thermostat was probably malfunctioning long ago as it always took a while before the car reached operating temperature. Now the car gets to 85-90C right away and I can see the thermostat open and stop the climbing temperature needle. Hopefully this will help my gas mileage, as it has always been lousy, in the high teens.