That is a "head scratcher". My guesses are:

1. A peanut masher for making crunchy peanut butter.

2. Nut meat masher.

3. Fruit masher for making jams.

4. A grain masher to prepare grain for making fermented beverages.

I have seen one....but trying to remember....the best I can come up with is a crusher for sorghum stalks to make syrup.
I have seen a demo of making sorghum syrup and that may be where I have seen that thing.

Found it! It's a butter worker!

Here is a link that might be useful: Butter Worker

The groove is a failed glue joint between planks, not part of the drain system.
Casey

It may have been modified by cutting the legs to make the tray level, or mounted on a table base, because the ones I remember were tilted. You put a bucket under the hole to catch the buttermilk, and just roll back and forth over the butter. It's way easier than doing it with the paddles.

Something similar can be used to press the lard out of cracklins when you have scooped and drained all you can.

No, the groove is NOT a failed glue joint between planks.
That is the way it was made.

Lazygardens, that may be true. Otherwise the buttermilk would pool.