These vintage adverts of skinless frankfurters and wieners are from the 1940s. Though the original Frankfurter (from the city of Frankfurt) and Wiener (from the city of Vienna) sausages were no doubt distinct, the nouns frankfurter and wiener (and hot dog / hotdog) have long been referentially equivalent; various groups of speakers have preferences for one over the other, and some speakers judge hot dog / hotdog to be the neutral term, while frankfurter is more formal in style and wiener more informal, no one takes them to have different referents.
According to Wikipedia, “skinless” hot dogs must use a casing in the cooking process when the product is manufactured, but the casing is usually a long tube of thin cellulose that is removed between cooking and packaging. This process was invented in Chicago in 1925 by Erwin O. Freund, founder of Visking which would later become Viskase Companies.
The first skinless hot dog casings were produced by Freund’s new company under the name “Nojax”, short for “no jackets” and sold to local Chicago sausage makers.
Skinless hot dogs vary in the texture of the product surface but have a softer “bite” than natural casing hot dogs. Skinless hot dogs are more uniform in shape and size than natural casing hot dogs and less expensive.