Although you will find "counter thresher", as in the following examples: (read on)

" The relatively popular manual reaping of MAIZE is more often replaced by mechanical selection of combined harvesters, especially in tropical areas. Combine harvesters are machines which clean maize in one operation, threshing and pre-cleaning. Opt maize cutting machine.
The combine-harvesters are usually self-propelled and have cutting equipment, a threshing chamber consisting of a sweeping threshing drum (with teeth), a permanent COUNTER-THRESHER, and paddle-placing equipment."
https://www.articleritz.com/the-advantage-of-maize-machine/

"Equipped with a rotating threshing-drum (with beaters or teeth) and a stationary COUNTER-THRESHER, these machines often have devices to shake out the straw and to clean and bag the Brain [sic!]."
https://www.fao.org/3/t0522e/T0522E07.htm
(Some sites refer to an 'adjustable counter-thresher')


I think your term refers to what is called the 'concave':

" Combine CONCAVES are used to filter grain and small straw in the treatment of grains such as rice, corn, etc. Among the components of a combine harvester, CONCAVE IS LOCATED near the front of the machine, UNDERNEATH A THRESHING CYLINDER. Together with the cylinder, THEY FORM A SECTION NAMED “THRESHING UNIT”, performing specifically the threshing function.
After being squashed against by the threshing drum, grains are filtered through the concave. This is the second function of combine concave, as it separates grain from its chaff (winnowing process)."
Description and diagram here:
https://thanhtrung.vn/en/threshing-frame-upper/combine-conca...


More here (believe it or not, while the text gets off to a rocky start, the site is Illinois English):

" The threshing chamber is the tunnel of working the valuable material for the not valuable plant matter. The threshing segment of the combine uses physical rubbing action between the steel drum and the grain. The proprietary design of your machine will include threshing lobes, rasp bars, open windows, cover plates and all manner of mechanical enhancements to deal with the particular grain you are harvesting. We have many video tutorials on the effects of CONCAVE type and it’s effects to the major grain types like soybeans and corn. We HIGHLY recommend you try cover plates in soybeans for example. The rotor surface of the CONCAVE uses centrifugal force and other grain to KNOCK KERNELS FROM COBS, beans from pods and seeds from seed heads pushing them through the slots or holes in the drum. The effectiveness of that rubbing action is proportional to the rotor/cylinder speed and the clearance between the threshing components and the surface of the CONCAVE. The size of your CONCAVES are a critical function determining effectiveness."
https://calmercornheads.com/corn-combine-troubleshooting/

However, there are at least two technologies for threshing corn/maize in combines, so there might be another term also.