This is a Mauritius Pre Stamp Full Letter.  It was sent from Bouganville to St. Servan by sea mail via Nantes on 2/25/1846.

What makes this letter so special is not merely its age and condition, but the postal markings it has.  First there is the NANTES TRANSIT on the front in RED.  There is a BLACK CIRCLE with PP for Postage Paid on the front.  On the back, there is a St. Servan Receiver in Black.

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.