[Jim McDougall]

These are intriguing weapons, and as noted associated with the various forms of Chinese martial arts, with these hudeidao becoming rather a trope associated with the notions of 'river pirates' in popular literature and media.
The 'river pirate' theme of course in not exclusive to China, but there it has become more well known due to US activity there, especially on the Yangtze River in the 1920s.
This was depicted in the 1966 Steve McQueen film "The Sand Pebble", which loosely referenced events involving a US Navy gunboat on that river.

While this topic is far outside my range, I became familiar with these 'butterfly knives' through Gavin Nugent, whose interest and knowledge on them truly piqued my interests.
Actually the use of these by martial artists (Shaolin, Kung Fu and several other forms) seems to have had the objective of injuring opponents rather than dispatching them. The actual knives used by 'river pirates' as I have understood do not have the hand guards.

The association with Indo Chinese (Vietnamese) guom is understandable as these regions were Cochin China and profoundly Chinese in demographic.

It would be fascinating to learn more of the history of the gentleman who owned these knives, and there are of course any number of possibilities where he might have obtained them. During the 'Boxer Rebellion' in 1900, of course, all manner of these kinds of weapons were brought out as souvenirs, and martial artists were literally full force everywhere, presenting dramatic and theatrical performances to intimidate and dissuade the 'foreigners' in occupation.