The designation for the robot as "model B9" is heard but once. This occurs in the second-season episode "The Ghost Planet" (episode 32, original air date: 28 September 1966).

During a scene in which the robot is being interrogated by the Supreme Prototype of all Cybernetic Machines, the robot is given the command:

"Identify yourself!"
To which the robot replies:
"Robot model B9, designed and computerized as a mechanized electronic aid for Earth voyagers engaged in astral expeditions."
This is the only time of which I am aware that the robot is referred to as "model B9."

Yet, in the second-season episode "The Colonists" (episode 54, original air date: 15 March 1967), the Noble Niolani imperiously and derisively asks the robot:

"What are you?"
The robot answers:
"I am a robot of the class M3, programed to provide information and support to all Jupiter personnel."

Later in the episode, the Noble Niolani appears at the Jupiter II campsite and asks the Robinson elders:
 

"Where are your weapons? You were ordered to surrender them."
John Robinson replies:
"What about our children?"
The Noble Niolani coolly answers:
"They have been taken captive — along with that repulsive creature class M3."
As an aside, Major West says to John and Maureen:
"They got the robot."

Both episodes were written by Peter Packer, so how can the robot be both B9 and M3? Is this an error in continuity? Perhaps, but there is another possible explanation. "Model B9" and "Class M3" are not necessarily mutually exclusive designations because a class and a model are different orders of being. It is tempting to speculate that Packer felt that, with "The Colonists," it was now time to give the audience a greater peak into the robot's history. The designation "Robot model B9 of the class M3" opens up the door for a richer and more varied taxonomy for the robot.

Of course, the "class M3" designation revealed in "The Colonists" helps to explain why the Japanese toy company Masudaya called its Lost in Space robot toys "Robot YM-3." I am unsure what the "Y"' in YM-3" means (perhaps the Japanese word for "class" begins with a "y" sound), but the "M-3" must surely come from "The Colonists."

To restate the initial question: "Is it correct to call the robot from Lost in Space "Robot Model B9"? The answer is "yes," but it might be better to call him "Robot Model B9 of the class M3." And, if you are especially peeved at your robot replica and at how quickly and completely his construction is emptying your purse, it would be even more correct to refer to follow in the black velvet footsteps of the Noble Niolani and refer to him as
"That repulsive creature class M3."