Champ didn't have a machine to make shooters. They made nothing bigger than the new old fashioneds in the 3/4" range, and none even that big before or after that run. That info comes from Dave McCullough and 2 different factory workers.
SMM only had a 5/8" machine all along until just recently, I was just at his shop a few days ago and saw his first successful run of 3/4" marbles. They were made less than a month ago. Before that they were all done on a 5/8" machine and 9/16" is easily in the range of what would come off that machine.
Don't worry, we all scratch our heads sometimes no matter how long we've been collecting. This one is tough. I lean Vitro with the one straight and one curved seam.