I don't know why "butt knives" became the standard term in English but they are known by different names in southern systems. They have been called wuh dihp dou, bau jaang dou (elbow knives), juk yihp dou (bamboo leaf blades) and even simply seung dou (double knives). I think some schools call them "hidden knives," as well. The names reflect either their use in pairs or their ability to be hidden up a sleeve while nestled against the forearms. These are primarily assault weapons designed for assasination, not self-defense.
Some are actually quite light and slender, some of the cleaver type and not all have the forward hook quillon or "D" guard. Hskwarrior posted some good photos a few years back showing some that were found in San Francisco.
No connection to Songshan that I've ever heard but certainly to some schools calling themselves Siu Lam. They seem to serve as symbols of resistance to oppression and probably reached their heyday around the time of the Taiping wars when it was well for anyone to be armed to fight even their own predatory militias.
"Look, I'm only doing me job. I have to show you how to defend yourself against fresh fruit."
For it breeds great perfection, if the practise be harder then the use. Sir Francis Bacon
the world has a surplus of self centered sh1twh0res, so anyone who extends compassion to a stranger with sincerity is alright in my book. also people who fondle road kill. those guys is ok too. GunnedDownAtrocity