The steel rod through the butt quite unlikely screws into the hooked tang, referred to above as "the engraved part at the back".

There is a hole at the back of tang that takes the hook of the breech, thus there is no where for the 'rod' to screw into. It is most unlikely that the rod actually screws into the back of the breech's hook.

What is likely, is that the rod merely reinforces the wrist of the rifle.

Hooked breeches are VERY common on English guns and less so on American guns of the same period, except for rifles such as the Hawken rifles of St. Louis as well as other 'plains-type' rifles of the period. The British design influence was strong in these guns - apart from the hooked breeches on many.

"Firearms of the American West 1803-1865" has a nice picture of an 11 bore single shot rifle made in San, Francisco you would swear came out of London about mid 1850's, complete with Alex-Henry-looking lock complete with drip bar, and of course, a hooked breech as on the double rifle of this thread.