There's guys on here that own original Armstrong rifles, alas, I'm not one of them. But, from what I remember, Armstrong used a domed head screw. Although the dome isn't as high as a modern dome head screw. Also, the screw slots wouldn't be real narrow, say like an English screw. In fact, the slot would be probably at least as wide as a modern screw.
Maybe someone with more personal experience will clarify this a bit better,
John
PS, Ooops, LRB beat me to it.