I trust lpeterssen provided enough leads to help you, so just to complete the picture:
1. This thermostat housing, to the best of my knowledge, has not been offered as a spare part, therefore it is not on the parts list, so I cannot find its Daimler part number. We could track the Bosch part umber, but it may not make sense.
2. By looking at the thermostat itself, that was and is a spare part and has the part number, I can see that the change was introduced together with M129 engine, so with 250SL and did not change for 280SL (early thermostat A 001 203 61 75, late A 001 203 9575).
3. They are not interchangeable. The old one was difficult to find, now I think I see it at SLS. The new one was available by several vendors, including Niemoeller.
4. As for the thermostats themselves, they are available at Mercedes, but the early type costs horrifying amount of money (ca EUR 700 + VAT). The newer one is just moderately more expensive at MB, ca EUR 120 + VAT than in other stores. I bought it from Mercedes as you do not have shipment cost, can return easily, etc.