These are not always used in conjunction with a bottom swedge. Artificers were issued a 1/4 inch radius fuller, but no corresponding bottoms. It seems that these were used to smooth out a forge welded joint where a piece was welded perpendicular to another piece.
My bad, Branson. I can see that use/purpose as well. I was just commenting on the general use of a fuller, and didn't get into too much detail.
They also can be used in conjunction with the face, the horn, or just about any surface the individual smith would want as a backing surface.
That's the beauty of blacksmith work, so many techniques and tools, and hundreds of ways of combining them together!
Mostly though, I was trying to get the point across that the tool
IS NOT a hammer....it just looks like one.
Although that thought is good for blacksmiths, because if a lot of people knew what these absurd looking "hammers" were intended for, none of us could buy them at a reasonable price!!!!