This is older base with red characters in the AAA-101 format that is still in use, but only with the letter N in the first position and higher. Plates lower than that have been replaced due to defective ink on the embossed characters. It appears that this base ran through WRZ-999, then the new series listed above in the reverse format of 101-AAA with black characters began.
The original plates in the AAA-101 series had blue characters, but they were changed to red because they looked very similar to the current Illinois plates at the time. None of those plates are still in use. They were replaced by the end of 1993.
Update from Nate Dee:
The WI DMV is again renewing remaining U, V and W series old style red on white base plates, at least as far as 2013. It's possible they will remain even longer. Originally when the new black on white base debuted, the DMV's website stated that all old bases would be replaced by a certain date by way of new plates issued at renewal time. 2010 seems to have been the last year this happened. The website has since been revised to say old plates will be phased out by new registrations and as customers request replacement plates.
And now, the latest update from Nate:
It has been announced that the replacement of the older red on white plates has begun again by way of new plates sent out at renewal. The special issue (1996-98) Sesquicentennial plates are also all being replaced in this re-issue. The replacement cycle is expected to be completed by fall 2014.
Dec. 2014 update from Scott Zilmer:
Can confirm that all red-lettered plates in the ABC-123 format have now been replaced with black-lettered 123-ABC series plates. The only red-lettered serials still on the road are expired. Some people are still running their red-lettered vanities, but replacements for all those have been issued. There are still a few of the older style kids plates with red letters that are in use.
Gary Fox - 15 May ’05