As indicated by the caption, this image shows an identification tag for the rendering of the fireplace elevation proposed for the Mackay mansion's library.
The caption accompanying this photograph reads as follows:
Although other work, in anticipation of the elimination of Harbor Hill's original library might have already started, the over-mantle design drawing shown above, with other plans for the 'stone room,' are dated 1904, two years after the house was officially completed
See Harbor Hill, Clarence Hungerford Mackay's Mansion.