Here is what 12020(c) says:
Large-capacity magazine restrictions - Calguns Wiki
-----------------------------------------------------------------------
Manufacturing non large-capacity magazines
Penal Code 12020 (c)(25) states:
12020 (c)(25) As used in this section, "large-capacity magazine"
means any ammunition feeding device with the capacity to accept
more than 10 rounds, but shall not be construed to include any of
the following:
(A) A feeding device that has been permanently altered so that it
cannot accommodate more than 10 rounds.
This means that one can manufacture non large-capacity magazines if the alterations to what would otherwise be a large-capacity magazine are permanent.
In sworn testimony, representatives of the California Department of Justice have stated that it is their opinion that the types of modifications presented in the failed "detachable magazine permanence" rulemaking would suffice to be permanent alterations to large-capacity magazines to exclude them from the law. The relevant alterations from the failed rulemaking include:
Continuous ribbon or ribbons of welding
A rivet fixed in place with epoxy