I imagine that your searches would have already revealed this:
http://outils-anciens.xooit.fr/t1172-Queue-dhttp://outils-anciens.xooit.fr/t1172-Queue-d-hirondelles.htm-hirondelles.htm
But in case they didn't, there it is! I've not tested Google Translate (I speak French) but if you need any help with the discussion, let me know. As best as I can tell it's used as a heavy plane (or slick) to smooth off the roughness left by the départoir (Froe). It is lancé (lit. thrown) so I suspect that your usage is correct as it's used a bit like an axe. According to one of the users it is sharpened to a single bevel, but I'm not sure if that should be bevel in, or bevel out.
