Possible ash pit?

[SteveAWright]

Hi all. taking out an old fireplace in my Northamptonshire, stone built farmhouse. Not sure of age of this part of the house but some parts date back to 17th century apparently. On removing the brickwork and part of the raised hearth, I've found this underneath. Is this likely to be an ash pit? The floor is solid but I believe there was a cellar below at one time, which got filled in. It's about 1cm above floor height, so I either need to remove it, or grind it down to floor level? Any thoughts on the best way to proceed?

Cheers all.

 

[Lower]

Looks like an old air vent to me. Its in the wrong place to be an ash pit.

Either way, i'd give it a smack or two with a lump hammer to see how secure it is. If it's loose, take it out. If it doesn't move, cut it flush with an angle grinder.