[MikeK]

The original gas line was terne coated seamed steel tubing. Terne is an alloy of less than 20% tin, sometimes 1 or 2% antimony, and the remainder lead. It quickly aged to a dull 20% brightness gray color. The quickest cover up that is about the same appearance and that will hold up without damaging the new line is a rattle can of dull zinc rich 'galvanize'. Not the expensive shiny stuff, the standard dull kind.

Of course, if you are into fine-point or want the absolutely untouched, un-restored, all original parts look, you can get a long length of today's electro-zinc plated steel line, set up a fixture and profiled tool bit to cut the appearance of the original seam, mix up your own terne, use an acid flux and a pad like an old-time plumber making a wiped joint, and wipe the terne onto the tube at about 700F. Then if you want the aged look just wait!