NTK O2 sensors

[john spears]

 I'm looking for info on what APPEARS to be an NTK oxygen sensor. It is used
in a Horiba A/F analyzer. It doesn't say NTK on it, just some numbers and
"Japan". I know it's a very good sensor and has proven very accurate
throughout the range of normal (and some abnormal!) air/fuel mixtures.
  The replacement cost is very steep, so I was wondering if anyone knows of
any reasonable (cheaper) alternatives, or any info in general about the
sensor.   It's a 6 wire device with the numbers 2251 1389 on it, and screws
into a normal 18 x 1.5 mm hole. It doesn't look much different than any
other O2 sensor, although I know it is.

          Thanks,
  John Spears