This is the compact version of the engine thermostat intended for all Boxer engines (excluding single carb ones). 

The compact version thermostat is a replacement of the split-type on found on the earlier Boxer versions; Power assisted steering system required extra space for the steering pump therefore a new thermostat design was required. As a result the compact style version is fully backwards compatible with Alfa 33/Alfasud/Sprint/145/146 models twin carb or fuel injection.

Year 2019 Update: these thermostats are now almost non obtainable as Alfa Romeo has stopped production. After market brands like BEHR, Vernet, BGA have also stopped production so in case you are lucky to find a thermostat forgotten in auto spare parts shops, DO get it.