2003 SL55. Received a P202C-008 P0128 error and have been advised to replace the thermostat. Trouble is the mbparts website has two parts named identically that will fit this car, so unsure of which to replace. Does anyone know?

By now you probably have found one, however... I just now saw your post, sorry... I would call a Benz dealership parts department with the car's VIN number and ask for price and availability of thermostat, then ask for the part number. Maybe call three or four different dealers doing this until you get them all to agree on the exact part number of the thermostat that their computers say is the one for your car. Phone calls are free and it's fun being on Hold... LOL.
Probably will get varying prices. Then you could shop around on the internet at reputable places not eBay, for a genuine Mercedes/Bosch part. That said, FCPeuro calls it a Genuine Mercedes Thermostat assy, part number 113 203 0275 and their price is $91.99
Good thing about FCPeuro is they guarantee parts they sell, for lifetime.
Once you have the belts off the front of the engine, check all the pulleys for wear and if your water pump is anything less than Perfect get a new water pump as well. An SL55 AMG has the M113(k) engine so I would believe the 113 part number.

Hey thanks man, big help. I think youre right bc the 113 part number is confirmed on several third parties, and its only the mb parts that has this other 112 part. Much appreciated!