Starting in 1992 with New Jersey's first issue of reflectorized plates, there were images (usually called holograms on this web site, but properly known as "3M Ensure Images") running down the center of the plate.
These holograms had "NJ" on top, and then a lot code indicating the date of manufacture of the reflective sheeting.
New Jersey's lot codes always started with "B" - see below for more details.

The reflective sheeting was supplied by the 3M Corporation.

In mid-2000, somewhere between the KCW and KFJ series, the holograms disappeared.
3M was still supplying the reflective sheeting, but without any lot codes.

In mid-2001, at around the LGx/LHx series, reflective sheeting from Avery started being used.
The new "Avery base" was distinguishable from the previous (3M-1) base by the "New Jersey" wordmark - on the Avery base, it is bolder and spaced out a little bit more.
The initial Avery sheeting had no identifying codes on it.

Starting in mid-2002 with the NCA series, holograms returned to NJ plates.   This time, the year of manufacture ("02" or "03") is on the plate - surrounded by a number of small circles in a diamond pattern.
The circles (maximum number = 12) most likely represent the month.

Unlike the 3M holograms, which always ran down the center of the plate, the Avery "year-mark" has been seen in the center, or to the left side, or to the right side.

In 2004, NJ terminated the contract with Avery.
There were serious problems with the Avery sheeting - it did not stick to the metal plate ("bubbling" has been seen on many plates), it faded to an off-white, and it just didn't hold up very well overall.

New Jersey went back to 3M sheeting, (we call this base 3M-2, to denote the 2nd go-round with 3M) and still uses 3M sheeting today.
BMH1 was the fist code seen on the "3M-2" base.

The new 3M sheeting still uses the better-looking "New Jersey" wordmark from the Avery base, so these two bases can be very hard to tell apart from far away.

Actually called the "3M Ensure Virtual Security Thread", this special image started appearing on NJ plates in 2009, and is more commonly known as "the helix".
It looks like the double-helix pattern we see in picutres of DNA.

As you look at this image on the plate, and move from side to side, the strands intertwine.
It's very cool.