I was curious so I contacted Jimmy John's. They serve two different dogs. The guy I spoke with was very helpful and had no qualms about answering my questions. The smaller dog is a skinless Hatfield's beef and pork. I was told that it's the same dog that you can buy in the supermarket and at Phillies games. I've had this dog at Tido's in New Brunswick and a place in Jackson, N.J. It's not good in my opinion. Just a bad spicing. It was rated by myself and others as the worst hot dog (at Tido's) out of 87 places that we visited in the summer of 2006.
The other dog pictured on Holly's site is a fatter dog made by Dietz and Watson of Philadelphia. It is a natural casing beef and pork dog that was served at Jimmy John's for the last 40 years. I remember having a smaller, skinless Dietz and Watson dog once before. It was at Casino Tony Goes in Trenton. A very good dog, but all wrong for an Italian Hot Dog, which is how it was served there. While it was the worst Italian Hot Dog I ever had, the frank itself was very good and would be good on it's own; especially in a natural casing like the dog in the picture.
The guy I spoke with tried to claim that it is a special dog made just for them. I was too tired to dispute this, and besides I was happy that he was kind enough to divulge the brand, which is more than a lot of people will do.
On a side note, a new place opened in Elizabeth called Valentina's Hot Dog Grill. Someone I know went and said the dogs were just ok; that Jerry's nearby is much better. I called and spoke with the owner who would only tell me that they used a skinless all beef dog and grilled it. When I asked why he wouldn't tell me the brand (which he admitted I could get on my own), he said that if I knew it, I would get it and make it at home. While I already do this, it doesn't mean I wouldn't go out for one. I can get any hot dog I've had (including Galloping Hill Inn's) out, but I continue to patronize these places.
I can buy Grolsch beer at my neighborhood liquor store to enjoy at home cheaper, but I still enjoy one at the establishment my daughter works at. But I digress.