did you check to see if you can purchase a 15 round magazine for your pistol directly from Glock? that would be safest...
i think this is what i read, see excerpt below.
http://www.njsp.org/news/pdf/rp-061807-13_54.pdf
it was a proposed change to state laws back in 2007, no idea what current law says, but you might be able to look it up here:
http://www.njsp.org/about/fire_ag2.html
I recommend that you be sure about current state law before leaving your house with a modified high capacity (greater than 15 round) magazine in this state. i have an old 10 round magazine for a 15 round capable pistol that has a permanent metal stop formed within the magazine (not spacers at the bottom) limiting it to 10 rounds. i'd say that's permanent (but i'm no lawyer).
"A proposed amendment to the definition of "large capacity ammunition magazine" states that the term does not
include an ammunition magazine that has been permanently altered so that it is not capable of holding more than 15
rounds of ammunition. The definition is further amended to state that an ammunition magazine, which has been
temporarily blocked from holding more than 15 rounds, as by a piece of wood or a pin, will still be deemed a large
capacity ammunition magazine. The New Jersey Supreme Court has determined that in N.J.S.A. 2C:39-1f, the term
firearm was defined "not in terms of operability, but in terms of what the weapon was designed to do." See State v.
Gantt, 101 N.J. 573, 583 (1986). A device no longer retains the characteristics of a firearm when "it has undergone such
substantial alteration or mutilation that the instrument has completely and permanently lost the characteristics of a real
gun." Id. at 590"