When trying to distinguish a long cornet from a trumpet, look at where the leadpipe widens out to accomodate the main tuning slide. On a trumpet it is directly after the leadpipe finger hook. On a long cornet it is about an inch down from the leadpipe finger hook. The picture on the left shows a trumpet leadpipe, the picture on the right shows a long cornet leadpipe. The trumpet is a 1959 10B Victor, the cornet a 1959 6A Victor. Since these instruments have coprion leadpipes and brass main tuning slides, the difference is a bit easier to see.