Howdy.  I build my first muzzleloader in 8th grade shop class,  I'm pretty sure you can't do that today. 
I have gravitated to big bores and BPCR. 
I have 4 Remington Rolling Blocks,  my most recent a 43 Spanish Carbine.  Which is fine cause I own a rifle in same caliber I reload and shoot. 
The barrel about 22" is missing the rear sight.  According to the hole in the barrel it was about an inch long with one screw holding it. It's much smaller than the one on my 43 RB rifle. 
It doesn't have a ring,  do you think it was cobbled together or originally a carbine? 
I'll try to load some pics. 
YippyKiYay

Here's another pic...500 kb!!! Holy cow I wish I'd kept my flip phone

Here's the carbine film length.  The right side  forearm had an old crack and I repaired it,  but haven't cleaned it up yet.

That is the standard hole spacing for Remington's export carbine.  The small hole is smooth for the locator pin in the bottom of the sight base. The large hole is threaded for the sight mounting screw.  There were a number of variations depending on the contract and caliber of the carbine.
photo is from a New York State carbine.  the flip up piece is cut slightly different than a .43 Spanish.