4200, Spyder and GranSport Rear Reverse Parking sensor (probably QP too) same at Alfa GT/147/156/159/166 available on Ebay for £15, not the £100 each from Maserati.
The pattern on the face of the sensor is different but you can still buy all four for £60 if it bothers you that much, you cant swap your existing cap over as you will break the internal antenna if you try this.
Same fitment and connector.
Alfa Romeo part number: 735393479
Here is the 4200 sensor note the pattern difference and the blue wire that you will snap if you try to remove the cap: