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They are his instruments, that is his servants.

They are his instruments, namely his servants.

And what about these other two:

They are his instruments or namely his servants.

They are his instruments or that is his servants.

Are they correct?
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Hi,
'That is' is an interjection, and requires appropriate punctuation to show that. I would handle 'namely' in the same way.
They are his instruments, that is, his servants.

They are his instruments, namely, his servants.

And what about these other two:

They are his instruments or, namely, his servants.

They are his instruments or, that is, his servants.

Best wishes, Clive
Comments  
Namely is not an interjection.