The ‘Le Butin’ Letters are some papers apparently left by a former pirate / privateer / corsair called Bernardin Nagéon de l’Estang, that allude to a number of Indian Ocean treasure caches. However, no detail from any of the letters has yet been verified with reference to any other source, so it remains questionable whether or not they are genuine.
The version of these letters given in the links was published by famous French treasure hunter Robert Charroux in 1962, though note that an earlier version appears to have been published in Revue des Deux Mondes during 1935; while a yet different version was published in Paul Fleriau-Chateau’s “Aventures en mer: trésors pirates corsaires et aventures vécues”.
However, because of textual inconsistencies between BN1 and BN2 (on one hand) and BN3 (on the other), there is a reasonable argument that the writer of BN1 and BN2 (who was apparently Bernardin Nagéon de l’Estang) was not the same person as the writer of BN3 (who would, in that scenario, be an as-yet unidentified corsair).
BN3 mentions the Apollon, which was a ship famously commanded by the French corsair Jean Francois Hodoul in 1798. The Apollon was originally a ship from Boston called the Apollo: here’s the crew list for its last voyage. When it was captured, its prize papers were lodged with the Admiralty (including the ship’s log!), finally ending up at the National Archives in Kew. Here are transcriptions of depositions left by three crew members from the Apollon:
Furthermore, the Apollon’s crew list (from its final voyage before being captured by HMS Leopard) might help identify the writer of BN3, if this was a different person to the writer of BN1 and BN2.
Note also that, Captain T. Surridge’s account of the capture by HMS Leopard of the Apollon is still extant and held at the National Maritime Museum (ADM/L/L100), though not yet photographed and transcribed here.
Finally, given that the “last” sea battle mentioned in BN3 might possibly be that of the Amiral Bailli de Suffren, here is a transcription of his diary covering the days of that battle.