You don't know if that is, or is not, an act of "Manufacturing." The reason is that California has not defined the term "Manufacturing" except for the pre-building phase of a firearm for which a DOJ Serial Number has not been issued. Please refer to Penal Code section 29180(a). Here is the definition:
"For purposes of this chapter, “manufacturing” or “assembling” a firearm means to fabricate or construct a firearm, or to fit together the component parts of a firearm to construct a firearm."
When you perform an SSE conversion, you do it by fitting the component parts of the firearm.
The key point about section 29180 is that its application is limited only to its chapter in the Penal Code. It remains undefined for other purposes in California law.
Your certainly are free to use the term "Manufacturing" in any way that you see fit, but if you chose to apply your concept of the term to California law, you're likely to be disappointed with the result.
When courts are faced with making a decision about the meanings of words, they tend to look at uses of the word in statute, and then adopt such uses, not as binding definitions, but as interpretations of their meaning. A California court looking to define "Manufacturing" for the purposes of PC 32000 would not likely query Mr. "Five.Five-Six" as to the meaning, but would more likely adopt the meaning given in PC 291890(a).
I'm not aware of any such criminal cases proceeding on those lines. IMHO, California DOJ has their hands full of cases being litigated and are not looking to add to the list.