Grate of some kind blocking flue

[emorems0]

This forum has been really helpful so far so I'm hoping someone will know what the heck is going on here... I posted before about plans to put a new liner in our masonry chimney for our newer wood stove. I'm cleaning the junk out of the clay flue through the existing chimney and it turns out there is a grate of some kind blocking the wood stove flue from extending down to the fireplace.

We had a chimney guy here to give us an estimate a week or so ago and he said that since both flues measured the same length from the top of the chimney, he assumed they were just filled with junk between the fireplace damper and the wood stove thimble. He said it would be unusual for the wood stove flue to not extend all the way down to the fireplace. So I was surprised when I started cleaning it out and found a grate/plate of some kind. It sounds hollow when I bang on it.

My question is whether this is some kind of past practice when building chimneys (maybe in the 70's) and what the purpose is... will I be able to get this grate out when I break up and remove the clay flues or is it likely embedded into the structure of the masonry chimney?
*two photos of the grate thing inside the flue looking into the thimble, then a could overall photos to show the general structure

 

[begreen]

Is the plate below the bagged thimble opening above the mantel? If so, someone may have fashioned a block-off plate in the damper area.

 

[emorems0]

Is the plate below the bagged thimble opening above the mantel? If so, someone may have fashioned a block-off plate in the damper area.

The grate/plate is about an inch or less below the bottom edge of the thimble, you can see where the inside edge of the thimble in the foreground of the grate pictures. There is about 24 inches between the grate and the fireplace damper. I'm not sure if the grate/plate is like a solid sheet of something or like a grate of some kind that used to be open. I scraped it the best I could with the tools I had, but it's still pretty well coated.

 

[bholler]

Looks like diamond plate to me