they are out there. there are several pictures in parker pages.
I have one a ten gauge early lifter short frame with non radius frame/ barrel on a #3 frame.

The one I posted about is also an early short framed 10 gauge lifter on a #3 frame.

my gun dates to 1878. has the wedge forearm

my guess was for brass shells? I don't really know. But she shoots just fine with modern shells as well.

the rebate is deeper at the bottom than at the top of the circle.

Many Jones underlever guns, especially Double rifles, has rebated breech faces. When the under lever was swung, the barrels were free to move forward.

Here is the photo that George sent me. Machined out recesses in the face of the standing breech.

Attachment 26950

Mine also dates to 1878, ser# 113XX. 10 gauge, wedge forearm, #3 frame & also has the tapered rebate which I assume facilitated opening and closing the gun. Guess there's more in the Parker book than we know of.

What would be the purpose of a rebated breech face?

I've seen rebated standing breeches on European under lever rifles and shotguns that moved the barrels forward slightly before swinging open. This is the first I've heard of Parker Bros using a rebated breech. Indeed the shorter frame facilitates the moving of the barrel away from the breech face but also requires the action to open further so the shells will clear the top of the frame for loading and extraction. Im thankful they got away from that design!

I too have a gun with the rebated breach face When I get home on tuesday I will take a photo of it, as John says mine is deeper at the bottom than at the top of the circle also Gary

I do believe I have seen English guns where the breech face was the opposite. And the barrels, when closed, would slide back and interlock with the protrusions on the breech as a final locking feature.