PASSENGER (BLUE BACK)
When NJ converted from the blue plates to the graphic plates in 1991/1992, there were some unused blue plate blanks left over.
Reflective sheeting was applied to the blue blanks, so we have some reflective plates with a blue painted back.
RX-589J has a regular silver back. (RX, RY, RZ came directly before UA.)
The cutoffs of the passenger plates are not exactly known - most of the U series may have used the blue blanks.
The U series was issued in late 1997 to mid-1998, out of sequence, and overlapping with the issue of the V series.
(No V series plates that I know of had blue backs.)
Did the use of the leftover blue blank plates have anything to do with the overlap of the U and V series ?
Hard to say.
UW-511S has a regular unpainted silver (aluminum) back.