I believe that on most rabbet planes the blade is a shade wider than the opening.
True. But most rabbet/fillister planes cut along a straight line. Only the
bottom of the blade does the real work.
I suspect you'd need a nicker ahead of the blade to cut a curve with
this type of plane. But having both sides curved allows you to always
cut "downhill" on the grain. You'd always work from each end toward the
bottom of the radius at the middle.
With the blade installed, the mouth looks better but still rather open.
After giving this more thought, I'm convinced this was a user-modified
plane. I doubt someone intended to add some ironwood as a new bottom.
I just found out that Scioto was a budget line for Ohio Tool. That might explain some things?
I agree this is another clue against a rabbet/fillister plane. A cost-conscious
tool line won't offer exotics like a curved rabbet/fillister. They just sell the
basic meat and potatoes in large volume.
But if a craftsman needed to modify a smoother for a special one-off job,
then it makes sense to grab a less expensive tool and go to hacking. It
may been used as a curved rabbet. Another good guess is that someone
had a trough or channel which needed a smooth bottom. The width of the
trough was close to the same width of the blade. By curving the sides of
his plane profile, the craftsman could slightly skew the plane while in the
trough and cut with less effort. I'd also guess that the trough depth was
exactly the same as the plane's inset.