I can answer this, but can't sauce it. If the state registers their voters via the DMV, all correspondants has a code that the post office machine reads identifying it's related to elections.
What they don't tell you is they also know what your party affiliation is by reading that barcode.
I was eaves dropping at a local DMV while waiting in the window, then lo and behold someone from the election department called into the radio station and confirmed nearly the exact same thing.
Again, no sauce but have no reason to doubt it. If you have a Drivers license, are over the age of 18 your vote can/will be stolen/suppressed as necessary and they control the entire process.