Is that more rare by any chance?
Hey Josh,
Not more rare, really, but a good indiction of age. The Baltimore loop was one of the last stops, before the age of crown caps. If you click on the Bruce Mobley link above, you'll see that all the examples he has are loopers, save that first tooled crown.
"Bottle Seal or Baltimore Loop Seal Closure, circ: 1885-1905,
Invented by: William Painter,
American Patent: September 29, 1885, Number: 327,099,
This stopper enjoyed moderate success, but was more popular in some areas of than in others. It consisted of a disk, made of a flexible material, that was inserted into a thin groove in the lip of the bottle. It was replaced by Painter's more popular crown cork stopper."
North American Soda & Beer Bottles - Beer Closures