Here are some closeups of the Rotring 1.9 and the Speedball C-4. That's the Rotring on the left.
The Rotring has a thicker nib and rounds over the top for smooth upstrokes. (This shot is straight on and it may be a bit hard to discern the very tip of the nibs. Do click on the picture for a better view.)
The Rotring has one supply slot while the C-4 has two.
And if you're new to calligraphy, as I am, be sure to check out yesterday's comments--Gretchen and Katharine give out many, many great tips!