Another hidden wall cupboard revealed...or old fire window

During the lime plaster repairs we came across a previously hidden wall cupboard, this is in the ground floor living room and also in the external wall adjacent to the window and next to the mahogany cupboard, so possibly a fire window. 

Opening exposed when plaster removed, slate was covering the hole.

Possible fire window, albeit there are bricks to the left hand side

The window opening will obviously be kept, we will insulate the space and block up and plaster in lime.