I was curious enough to call my contact at Smithfield. She put me in touch with the fresh meat group and this is the answer I got to the question "What, if anything is the difference between a collar butt and a Boston Butt?"
A collar butt is the European equivalent of a US Boston butt, BUT
European processors cut or "break" the animal differently than we do. US processors typically break the animal at the 1st rostral rib. European processors typically break them at the 4th rostral rib so a European butt (Collar or Boston) will have more loin meat on it and usually be larger than a US Boston. He told me that some US processors have started cutting butts European style, trimming out the forward portion of the shoulder and labeling those as collar as opposed to Boston to differentiate them. Then again some US processors have simply started using the collar designation interchangeably with Boston.
In other words "it depends"