You are quite correct. When I was in my formative years I used to go to Wood Green Jazz Club in the Fishmonger's Arms every Sunday evening to listen to Alex Welsh. I was told by a friend that he played a Trumpet Cornet and I went to Lewingtons to buy one, thinking that it was a hybrid of some sort. I was then put right by the salesman. There is no such thing as a Trumpet Cornet; it is a Long Model Cornet and has the same internal tubing bore dimensions as a Short Model (or Shepherd's Crook) Cornet.