Twist-to-tighten nibs were invented around 1838 and the initial form of the band on the nib iron mimicked that commonly seen on wedged nibs of the time. Because wedged nibs don't use the cinching action that twist-to-tighten ones do, they benefited from having a more tube-like flare to the band so there was more contact area to wedge against, sort of like wedging an axe handle relative to the depth of the eye and the effect that has on the security of the fit. This is an example of an early wedge-fit nib.
Side view, showing the flare:
Looking into the band. Note the fairly thin wall thickness.
Top, showing the projecting tang of the nib iron.
Looking at the junction of the loop and grip. Note that the wood is carefully shaped to mate snugly against the shaft of the snath, and there's no space in the top of the loop for a cinching action like on a twist-to-tighten. It's just a neat loop.
You can compare and contrast the overall form of the above wedge-fit nib against the form of the first-form twist-to-tighten variety shown
here. Note how the nib block (the brass ferrule) follows the same shape that the wood itself does on the old wedge fit nib. If taken together with the wooden grip, it strongly resembles the shape of the grip on the wedge fit nib. The band is a lot narrower, but still flares at the bottom.
Later wedge-fit nibs on cradles often have a more simplistic form to the band, having drawn much of their shape from the then-current form of the bands of twist-to-tighten nibs, which were avoided on a lot of cradles because of the extra long length of the grip making such an arrangement trickier to make work right. The long grip was needed for balance purposes due to the weight of the cradle, and it was used off the ground, unlike a grass scythe, which is why cradle snaths often seem so short.