This is a Mauritius Pre Stamp Full Letter.  It was sent from Port Louis, Mauritius to Marseille, France by overland mail on 12/1856.

What makes this letter so special is not merely its age and condition, but the postal markings it has.  First, there is the BLUE MERCHANT'S HANDSTAMP on the front.  Then, there is the RED PACKET LETTER on the back.  It was endorsed to le overland.  It's a correspondence among merchants thanking the French Merchant for satisfactory prices of sale.

This cover likely transitted through Mauritius, but no marking shows as such.  Nonetheless, a VERY RARE Origin from an early date to Brittany.

Please take care as this cover, while it is in fantastic shape, is still over 100 years old.

This item is over $100 USD so I will ship it using Priority Mail in the US, and Registered Mail Overseas.  This is to ensure that it reaches you in a safe and timely manner, and includes tracking!