From all I can find,,this likely would not be from WWI !! James Montgomery Flagg donated 46 paintings similar to this for the war effort !! In 1917 and 1918 there were over 4 million of his paintings made into posters for WWI !! All were done on heavy grade paper stock as far as I can fiind !! In my mind since there were metal shortages during both wars it would not be likely that the army would have used metal for recruitment signs when it was needed for the war !! At some point at the end of WWII they did make some metal signs and continued using them till 1978 !! My opinion is that this sign is more likely from the Viet Nam era !! Army surplus stores would have a number of these since they would be replaced when they started to rust and wear a bit !! If you will notice on the link you posted,,he says that the sign is “rare” at the bottom in the description, yet at the top he has “in stock” under the price which is a ridiculous amount !! “In Stock” would mean he has a bunch of them so can not be a “rare” sign !! My hubby is U.S. Army retired so I know how the Army purchases and they always buy anything in bulk !! That would include recruitment signs !!
I will continue to see if I can find any additional info on this sign !!