Block II Command Module
In 1964 North American Rockwell Gave NASA The design of the Block II CSM to replace the Block I. Many design flaws had been discovered while extensive inspections had been carried out. There were also extra needs on the Command Module That could't just be added to a Block I.
The Block II also featured the docking equipment for the LEM as the block I was designed just for Earth Orbit and therefore didn't require any. Apollo 4 and Apollo 6 both featured an uprated Block I with many Block II features, However it wasn't until Apollo 7 when the first Block II CSM (Spacecraft 101) was used.
Design changes continued to take place throughout the program as studies and analyses progressed, as hardware failure occurred, and as new requirements developed. Major modifications were made for the final three missions because of expanded requirements for scientific data acquisitions from Lunar orbit. While these modifications were being implemented, the investigation occuring from the cryogenic oxygen system failure experienced on Apollo 13 dictated additional changes.
Manned flights of the Apollo Project only used Block II Command Modules after the Apollo 1 tragedy.