I agree, this looks like an error by the dial manufacturer. The Machine Cancel Society's "Volume 1, International Postal Supply Co. Hey-Dolphin Cancelling Machine Model HD-2" volume from 1995 records North Branford had an HD-2 in use 2/2/1959-1960, which I see I own the EKU example.
Your 1958 error dial may be what came with the machine when it first was deployed there, and quickly corrected. Interesting that the killer was also replaced. It is hard to assess whether yours is philatelic or not, given what you show. Errors like this are not very common, but seldom command much premium.
The HD-2 machine looks like this:
A simplified description: The stack of raw mail is placed at the back left, the crank turned, and the stack of cancelled mail accumulates at the front right.