I know this is not what it is but I saw a huge thing like that. It was used to regulate the flow of water down a chute on a grist mill. I don't know...It's definitely interesting. Good find!
Appears to be used for regulating the flow of something. Perhaps seed, or feed? Could have something to do with a grist mill, maybe?
I seem to recall some form of that device used as a regulator on an auger used to move seed into a silo. Been maybe 40 years since then, so I don't recall it in detail.
It looks like and adjustable flowgate for a gravity-feed something-or-other. The curved edge probably fit in a chute and the notch allowed a set volume to flow. The adjustable gate would enable an increase in rate of flow. Not exactly sure what kind of material, though...probably grain.
Seeder, Sower, Planter, flow adjuster all seem on the right track. Size does seem to be only about 3" wide though so maybe it is from one of the old early 1900's hand seeders or Cyclone broadcasters as some were called. Couldn't find a good picture of the actual flow adjuster but here's what one looked like.