Although a published advert may be called original technically an original advertisment means an original drawing or draft. When people call such an item an original print (one taken from a magazine for instance, or a poster) they are comparing it with a modern repro.
With steel signs it may be difficult to tell if the sign is "original print", it may be best to leave the word out. Of course some advertisements may not have been steel signs in the first place (or postcards, or even posters so they are reproductions but not reproductions of something that was available at the time). It may be safest just to say "this is a version of a poster advertisement" when no date can be found. You should also say that the information given is true to best of your knowledge.
Various collectable names include Guinness, Coca-Cola, Pears Soap and Railway posters. Having famous designers such as John Gilroy and Andy Warhol would make the advert worth more.