What is this? Fireplace covered by panel since ~1970

[Newbie2]

I recently removed panel that was covering an old fireplace. I believe that it has been covered since the 70's.

Inside there is a large stone/concrete? block with a small channel in the top.

The fireplace on the other side of the chimney is full functional but has no iron insert.

I would love any thoughts or suggestions as to what it might be? If it is an object lodged in place? Or just cast concrete to block up the fireplace?

I have hit a dead end...



(This is in a rural cottage in Hampshire)

 

[Gothichome]

Any chance there was an external stove outside of the fire place. The curve allowed the stove pipe to go up the chimney the rest of the cement is just acting as plug.

 

[CzDuck]

I think what @Gothichome suggests would have been my guess too. Some way to block up the fireplace once a stove was sat in front of it, with a pipe off the back up the flue.