Flattops - looking at the photo to the left it's not difficult to guess why these pens are called so today.
In 1924 Sheaffer introduced their new line of "Radiate" pens. Radiate was Sheaffer's trademark name for celluloid, a new material for fountain pen producers in the 1920s. Along with the introduction of the new material, that also made it possible to produce fountain pens in different colours, even marbleized pens, Sheaffer introduced the "white dot" promising a "Lifetime" guarantee on the their premium line of pens. When you know what the white dot means, it looks quite attractive!
Both the black and the green mottled pen shown are senior pens, that is the biggest size offered. Nonetheless, the black pen is a tad longer and larger than the green pen, but the difference is only millimeters, 13,4 and 13,6 respectively. The green pens are very prone to fading and discolouring, so as a matter of fact the green one shown in this page is a very good specimen for its vibrant colour.