California Code, Penal Code - PEN § 29125

(a) Each firearm a licensee manufactures in this state shall be identified with a unique serial number stamped onto the firearm utilizing the method of compression stamping.

(b) Licensed manufacturers who produce fewer than 500 firearms in a calendar year within this state may serialize long guns only by utilizing a method of compression stamping or by engraving the serial number onto the firearm.

(c) The licensee shall stamp the serial number onto the firearm within one business day of the time the frame or receiver is manufactured.

(d) The licensee shall not use the same serial number for more than one firearm.