Pill locks pre-dated percussion caps as we know them, then came along "tube locks" where the priming compound was inside a thin copper tube - the tube laid in a tray like a flinter's pan, with one end facing the vent. This was a European invention. There were very long sided caps as well, for very long nipples (nipples were called tubes in the early 1800's). A very long cap might have been used in a rifle such as this one, for ease of 'capping'. I do not know how you'd get the cap fragments out, though. Perhaps that is why guns such as this are rate and in good shape?