Yes, it does look like a very small rounder. Larger sizes made spindles for chairs or handles for tools. This looks like it would produce something at about 1/4" so presumably for dowelling. Veritas make a modern equivalent.
I don't think it's a commercially made tool. Assuming that the name Greenslade is on the iron, with the letters curved round, that looks like a bench plane iron, adapted, mounted on a v-block with a hole drilled or filed on its inside, larger on one side than the other, tapered on the larger side, then parallel. Neatly done though.
What happens if you cut some slightly larger wood and wind it through? Do you get an evenly sized dowel emerging?