Unknown use. Possibly a mold for bricks/blocks? Found in central South Carolina.
pics
Would be making kind of large bricks but I have seen similar boxes only a little smaller for making clay blocks, but not sure what was done with them. But this one looks a little complicated, not sure what those extra pieces are for. I suppose it could be used to form some sort of cored brick with holes for reinforcing rod.
Are brick molds usually covered on top? Seems like it would take a long time to firm up.
They are only used to shape the brick, the bricks would then be removed after shaping and laid out in an initial drying area, and then to a kiln. However, as I said above, I’ve seen clay blocks made for some other reason, which I never knew or don’t remember, but I don’t think they were heading for a kiln. I just don’t know what would be done with a block of unbaked clay, it will readily absorb water and lose any compressive strength it might have.
I think this is something more complicated. Also note the hole and the channel at the bottom. Maybe that’s for water squeezed out to drain, but if the box had wet clay in it then that channel would end up filled with wet clay. I think this thing is some kind of press but no clue for what.
It’s a Thingamajig! 
Sorry, but you are wrong. It may appear to be so but it doesn’t have enough reticular flanges . Definately a Whatchamacallit.
Calling a doohickey a thingamajig is fightin’ words where I come from.
I think it might be for compressing sawdust to make fire logs. There’s a pretty similar looking device here:
The spindles are too irregular to be for brick making
I was thinking about peat, but even if not specifically for sawdust I think you’ve found it’s purpose. The pictures in the OP show a yoke where a lever could be used to compress the material. The way the top and bottom have to mesh would allow easy removal of the compacted material instead of having it enclosed on 5 sides in the bottom of the box.
I’d agree - it’s definitely for compressing something - specifically something that needs to drain while being compressed, and probably to dry out after compressing (the two rods seem to be for creating ventilation holes to aid drying of the compressed block, I think.
Seems too large, too ricketty and too elaborate to be a brick mould. Ultimately, if it was for bricks it would need to be filled, pressed, turned upside down with still-wet clay and keep its shape onto a drying stack, which doesn’t seem ideal.
I thought some sort of press - cheese? tofu? The channel looks like it was draining liquid from something fairly wet, more wet than brick-clay would be. Its something that needed to be kept compressed for a while [locks on the uprights], so not plastic enough to hold its shape immediately.
The pictures in the OP don’t look like any brick molds I’ve seen.
When I was a kid, we excavated some old brick molds from the clay pits behind our house (they had lettering molds on the faces that helped up identify them). I cleaned and waxed them and donated them to the local historical society. I was happy to see recently that they still have them – they weren’t lost or misplaced.
They looked like this:
https://www.penobscotmarinemuseum.org/pbho-1/collection/brick-mold
Since they had to make a LOT of these (they’d fill the slots with the clay mixture and let it bake out in the sun until dry enough to take to the kiln), they tried to keep them as simple as possible.
That’s not to say that your device might not be some sort of extra-special mold for specific bricks, but it’s not a standard brick mold.