Ian,
can't remember to saw such a koshimaki on an suji bachi. The shikoro is damaged but real while I'm not sure about the helmet itself. Looks very odd to me.
Rob,
like Steve and Justin pointed out. Almost impossible to assign the mon to a certain family. Can't also find a match, but I think it's called "Sumikiri-kaku ni Tsuta".
Josh,
shape is shobu-zukuri. I assume Shinto and made this way. Difficult to judge from the pics if it's worth a polish. Seems that someone starts the work and decided to leave it alone. Otherwise looks quite healthy......
Rob, a considerable amount of yoroi bitsu in more or less good condition are extant. Most of them from the Edo period. Only a few have been preserved since Muromachi times. To be honest, I only saw one such for sale, so far (around 200.000,-JPY)!!!
Eric, it depends on the punched proof firing marks under the barrels and the "look box" (don't know the right term...). It is often a double crowned "U". In case of the double barreled pistol pictured above, I think it's def European!
Forgot to mention, that the barrels screwed on!