I remember reading that as well, if i recall correctly it was down on the bottom somewhere. maybe remove the bottom louver and look for the plateNo idea never measured. Are you sure nothing got closed during the lasts cleaning . I read a post on here with a Opel, once guys Opel would not put any heat out of the louver. I guess his had a plate that closes off the louver when fire place isnt in use.
I was trying to find the post before I mentioned to better explain the scenario. But yes check it