The Baltimore is the lead ship of the class of heavy cruisers unrestricted by pre-war Naval Treaties. The first 4 ships (CA68 Baltimore, CA69 Boston, CA70 Canberra and CA71 Quincy) were laid down in 1941 and commissioned 1943. They can be differentiated from the later class-ships by the characteristics of their stern: flat transom, with two aircraft handling cranes and a quad 40mm tub ofset to port.
Later Baltimore CAs can be identified with single aircraft handling crane along the centerline, with dual 40mm Bofors tub abreast it (the rounded transom hidden under these).
http://en.wikipedia.org/wiki/USS_Baltimore_(CA-68)
http://en.wikipedia.org/wiki/USS_Pittsburgh_(CA-72)
http://en.wikipedia.org/wiki/Baltimore_class_cruiser