This is a Mauritius 1919 Registered Cover.  It was sent from Port Louis to Buenos Aires, Argentina on 1/23/1919.

It has a number of interesting postal markings, including a London Transit dated 3/18/1919, and a Buenos Aires Receiver on the back.  It also has Mauritius and London Registered Marks as well.

What makes this cover interesting is the usage of the Mauritius SG 183, 3 Cents Green, SG 184, the 4 Cents Green, SG 187, the 8 Cents Orange, and SG 198, the 12 Cents!