Hi Chuck, the plate shown is 1957. Previous "square" NJ plates were renewable with steel, then aluminum year tags. In NJ in '57, this style (standardized 6x12 size) was introduced, with the two stacked letters and four digits. They were also used in 1958 and early 1959. A decal on the windshield validated it.
In mid '59, NJ went to a new color (black on straw) and format changed to three letters followed by three numbers. Some of these black on straw plates are still seen on the road today, as they are still valid to use !
Looking for photos, etc from 60s era Shell-A-Rama gas station and Pal's Diner, Rt. 17 Mahwah, NJ
& US or state highway signs, shields, route markers