There were three different reverse hub designs use in 1868. This is the second one known as the hub of 1868 and it was only used in this year.
For the Hub of 67 and the hub of 68 the working dies were made from the master hub and not from working hubs. When the Master Hub wore out a new one would be created. Beginning with the Hub of 1870 they went to the Master Hub, Master die, working hub, working die system that the use today. Apparently the Hub of 68 was consider a failure, or possibly its creation was just a fluke, but only one Master Hub of this design was created and as it progressvely broke down through use the dies created from it can have no broken letters, one, two, three, up to six broken letters. (We know there was only one master hub because the broken letters come in a certain sequence even though the come from differnt dies. This letter first, then that one then the next etc. This is in contrastt to the hub of 70 which also comes with broken letters but the which letters break and when happens at random indicating the come from multiple hubs.)