The cancel was made with a machine similar to this one.
http://catalogue.postalheritage.org....0machine%27%29
The one shown is a Krag, while the one on the GPC is probably an international. The operation is essentially the same. There's a spining ink pad, that inks a spinning steel die. The wheels at the right push the cards or leters into the space between the spinning die and a rubber roller. The die grabs it and prints the cancel as the mail is pushed through to the other side.
The tips of the wavy lines hit first, the circular part with the town last. A nice solid cancel will indent the paper a bit. The mail often slips slightly, and the die is very well inked. The sloppy bit at the right side of the circular part is a bit slip a bit sloppy ink, and darn hard to fake unless you make a machine.
The number to the left of the circle is the machine number. A small town might have one machine, Chicago had a whole roomfull. If I was going to the trouble of faking a cancel it would NOT be a Chicago machine cancel......Not when a card like this can often be had for pennies with a perectly good cancel already on it. (Plus the postal inspectors dislike cancel faking nearly as much as stamp faking, and there's no statute of limitations)
Steve B