That drum is NOT stained from gas residue, more than likely from many years of heavy use along with being in a slightly damp environment. I have a 1963 RCA Whirlpool Imperial dryer that has a stained drum and it’s electric, too.
Glenn I believe you are correct on the serial number. It looked like a 2 to me but your picture is better and I believe it is a 4 making it built the 48th week of 1964.