It sounds legal to me, but it is a tough call without having the rifle in my hands to make sure. In California, the model name has to be EXACT for it to be banned. So for example, a Steyr Aug is banned but a Steyr AUGA3 is not banned. So if it is marked 'AK47S' then it may not be legal, but if it is marked 'AK-47/S' then it should be good to go - so the EXACT marking is important and probably why the buyer wanted to make sure it was marked the way it is. I know that sounds silly and downright stupid, but it is the law here unfortunately. Ultimately, if the receiving FFL is willing to accept it, then I'd say you're good to send it and it will fall on them if they sell it to someone who can't take possession of it (there are some people in the state who are exempt from these lists and can purchase these banned items).
For future reference, for you and anyone who comes across this topic, the list of firearms banned by name can be found searching for the Roberti-Roos Assault Weapon Act of of 1989.
Wikipedia link to the Roberti-Roos Assault Weapon Act of 1989
Furthermore, the term "series" in the act was later clarified by case law. The courts held that the state couldn't just call something an assault weapon for being similar or close. It had to be EXACTLY as it is on the list and a code of correction was added defining what firearms were in the "series". Our penal code numbering scheme has changed since then, and I can't find the new section, but you can find the list in appendix B for AR and appendix C for AK series weapons on page 2 of the Calgun's Assault Weapon Flowchart PDF.
Calgun's Assault Weapon Flowchart PDF