JP mixed up two separate sequences of plate numbering, methinks.
The "E" and "NF" and "JX" plates that the OP inquired about are what
are called "county clerk" plates, issued at the discretion of the local
county clerk.
Here in Monroe County, the available prefixes are M, MC, MM, MN, MR, 1M,
2M, 3M, 4M, 5M and TU (the latter a long-ago favor to employees of the
now-defunct Times-Union newspaper, apparently.)
And if you know the right questions to ask when registering your vehicle
at one of the DMV offices operated by the county clerk (which is all of
them in Monroe County these days), you can get one of those plates at no
extra charge, as I did when I moved back here in 1997.
Funny thing with that - it turns out that my chosen plate, M401, should
never have been issued by the county clerk's office, as that same sequence
had ALREADY been issued as a vanity plate elsewhere in the state. And would
you believe it took more than five years before Albany figured that out
(when the "other" M401 needed his/her plates reissued) and made me change
plate numbers?
(Part of the problem was that, up through and including the Liberty plates,
the state stamped the full sequence of clerk plates for each county and
shipped them to the appropriate county clerk, and only THEN sent out a list
of which plates were duplicated in other series - primarily as vanity plates -
and should be pulled. It seems that nobody at my local office got that note
and pulled M401 before it could be issued to me. With the new blue-and-white
plates, the county has to order new clerk plates individually, providing
an additional check on the system; apparently, the state still issued them
M1 through M199 readymade, as I was able to acquire two - consecutive, even -
plates from that series to replace my old M401 and my wife's 2M802.)
In any case, county clerk plates consist of a one-letter, two-letter or
number-letter prefix followed by one, two, three or four digits, never
(in my experience) higher than 2000. They never duplicate the old six-figure
Liberty plates, which started out as LLL NNN, then NLL NNN, then LNN NNL.
(The first three figures on those plates WERE county-based; here in
Monroe we started off with a lot of Mxx NNN, then Ixx NNN before they ran
out of numbers. The gold-and-blue plates that preceded the Liberty plates
were also county-based, but in NNN LLL format and with the last three
figures unique to each county. The current blue-and-white plates are
apparently being issued in a statewide series; they started at AAA 0000
and are currently somewhere in the late Cxx range.)
But back to the county clerk plate: they definitely seem more common in
some areas than in others. They're quite frequently seen in Monroe; I rarely
see any downstate, and wouldn't even know what the prefixes are for some
of the counties down there. It really does seem to be at the clerk's
discretion as to how they're issued.
s