Combines adjustable chopper concave

What is a chopper concave, let alone an adjustable oneIJ

the chopper concave is the bar with the knives on it that sits under the chopper.

I found the following available, they all look good:Straw chopper knife kits: Clever Tech Inc. Jesup, IA 319-827-1311 Clinton Ag Service Clinton, MN 320-325-5203 www.maxminn.com_clintonag NooWay Ent. Currie, MN 507-859-2089