what is this

[elk killer]

was on fake book
never seen anything like it

[Old and Grumpy]

Is that the bolster or breach plug? Or both. If it is the breach plug It would be hard to clean.

[Steeltrap]

Is that breech plug supposed to be a modified one to accept a center fire cartridge?  Doubtful.....just a SWAG

[Old and Grumpy]

A attempt to get a more even ignition ?    What is the gun?

[Mattox Forge]

That is probably a bullet expander. I imagine that that rifle was set up to shoot heavy conical bullets fitted with a plug, box wood or so edging hard that would expand the bullet into the rifling when tapped by the ramrod.

Mike

[Hawken2012]

Looks like a Tige / "Pillar" Breech to me.  You would load a conical style bullet and hit it hard a few times with the ramrod to drive it onto the pillar and expand the bullet into the rifling.  Mid 1800's European military technology that was also used in some civilian arms.  A special jag with long hollow center was used to slide over the pillar to clean it. 

[Bob Roller]

That's more than likely.I have heard of this but never encountered it,Expander for a long bullet.At the time they didn't realize a log heavy bullet will upset/expand upon the detonation of the powder. and the expander was never really needed if the bullet was full land diameter.
Bob Roller

[Daryl]

Yes, that is a "Tige" breech. "FIREARMS OF THE AMERICAN WEST" addresses these as some military versions were purchased by each North and South armies from French and Austrian Gvts. during or before the Civil War.
The  normal "practice of "3 throws of the rod" to seat the ball paper ctg. onto the powder worked to expand the undersized bullet into the rifling so no further lessons were required for loading g them.
This military loading drill of "3 throws of the rod" is the reason many people still do this. With loose "fouling loads", I understand the reasoning.
Another "expanding bullet into the rifling breech a was the "Delvinge" rifle that had a breech plug with deep cavity that held the charge of powder, but had a smaller diameter than the bore of the rifle. Went the bullet was dropped into the bore, it rested on the shoulder of the breech plug. 3 throws of the rod, expanded the bullet onto the rifling.