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I am looking at getting a wider field of view with my raspi cam. I found a solution using M12 lens mounts and M12 lenses. In the process of fitting the M12 lenses, I am asked to remove the stock lens that comes with the raspi cam. 
This is the stock lens removed. Can I not replace this with a similar standard/sized lens?
Coming back to the main question.
What is the lens size/standard's name?
I doubt there is a definite answer to that. This lens is an OEM device and to my knowledge the Foundation's documentation does not provide any details about the lens, it's holder and their mechanical interface.
Why am I asked to move to a different standard?
I would contribute that to the availability of standard lenses of a different size, e.g. M12. An alternative is always to use add-on lenses that do not not require to remove the original lens, such as these
Lets look into known details anyways, though that might include data of the PiCamera V2 instead of V2.1 (let's hope and assume they are the same):
References:
