Vietnamese talismanic knife?

[blue lander]

Just won the auction for this. The seller thought it was Vietnamese from the early 20th century but who knows how he came to that conclusion. It's only 18.5 cm long so it must be some sort of amulet or ritual knife?

The wavy lines on the blade look a lot like the marks you see on Thai dhas and whatnot. I wonder if they're writing or just decoration? I'll clean it up a little when it arrives and take better pictures.