CPC 12090, as mirrored from http://law.onecle.com/california/penal/12090.html on 2/26/2010

Any person who changes, alters, removes or obliterates the
name of the maker, model, manufacturer's number, or other mark of
identification, including any distinguishing number or mark assigned
by the Department of Justice on any pistol, revolver, or any other
firearm, without first having secured written permission from the
department to make such change, alteration or removal shall be
punished by imprisonment in the state prison.