Two models have JG serials. Xmetal/Gold And Polished/G30. and the serial overlap BUT have a gap in the releases. They should have been JG and JPG. but best way to tell which is which is not the lens or the finish (as we know they can be changed) best indicator is the stem type:
Xmetal -Gold = JG Straight Stem
Polished - G30 = JG Hammer Stem