Google Maps is a bit misleading. As far as I know there is only one No. 49 on Fitzwilliam Square, the one on the West side (with the white door in Google Streetview). The numbering in the Square goes from No. 1 on Fitzwilliam Sq. East, through Fitzwilliam Sq. South and West to No. 69 in Fitzwilliam Sq. North.