It seems the numbers for the square where combined sometime between 1852 and 1862. Earlier directory listings (i.e. 1834 to 1852) show Fitzwilliam Sq. East with numbers 1 to 17, North with 1 to 17, South with 1 to 18, and West with 1 to 17.

The first listing I've seen so far with the combined numbers is 1862 - which has numbers 1 to 17 assigned to the East side of the square, 18 to 35 to the south, 36 to 52 to the west, and 53 to 69 to the North. This is the system still in use.

some details for that address :

1834 - 4 West, Lady Frankfort
1840 - 4 West, Lady Frankfort
1850 - 4 West, Lady Frankfort
1852 - 4 West, Lady Frankfort
1862 - 49, Capt. Henry De Montmorency
1868 - 49, Capt. Henry De Montmorency

I can check for other listings if that's of any interest...



S.