Fascinating stuff, Michael! I'm especially captivated by the few I see where the user seems to be gripping the butt-stock beneath the crook of his arm - gets me wondering about "jezail grip" again, y'see.
Interesting also is the subject of sights. I recall a conversation on the Nihonto Message Board, in which I asked essentially a similar question to one mentioned earlier: "Why put fore and backsights on something so inaccurate as a smoothbore muzzle-loader?" The answer has yet to be found, but one fellow suggested that it might be partially explained by the near-constant Japanese desire to improve on their wares, no matter how fractionally. I'd certainly call putting fore, intermediate and back sights on a Tanegashima an improvement - and a fractional one, at that.
Might we be seeing something similar here: early European gunsmiths (at their own impulse, or the instigation of their buyers) attempting to improve the breed?