"SKS with detachable mag" was primarily targeted at the Chinese imported SKS that could take AK magazine, not quite exactly the duckbill magazines. however, using the duckbill mag will also fall into this catagoty, not to mention Federal 922(r). So... If you have one of those SKS that accept AK mag, you are suppose to turn those in in the 1999 buy-back program (Can't even register it); but if you modify your SKS that originally have 10rds fixed mag to the duckbill detachable mag, not only you are violating California law, you are violating Federal law as well. And from browsing the forum (check in AK-47 section of this forum, under SKS), there really aren't that many of US made SKS parts available to make your gun compliant to the 922(r), so keep your SKS the way it is would be the safest, legal-wise.