See http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=48710&item=5718730 898

Pair 8122 electron tubes made by Burle, USA. New Old Stock, never
used, perfect condition military surplus stock. Both tubes have
matching GREEN color code. Date code 97-43

Current production Burle 8122 list for US$415 each. For further info
see Burle website http://www.burle.com/cgi-local/shopper.cgi?key=8122&preadd=action

Explanation of color code:

The 8122 tube manufactured by Burle Industries is measured so that the
correct color code can be applied. With 700 vdc applied to the plate
and 250 vdc applied to the screen, the control grid bias voltage is
adjusted for a plate current of 185 ma. The following control grid
bias voltage ranges determine the tubes color code based on the
operating conditions above:

Control grid bias voltage color code

-8.0 to -10.9Â*Â*Â*Â*Â*Â*Â*Â*Â*Â*Â*Â*Â*Â*Â*Â*Â*Â* red

-11.0 to -13.9Â*Â*Â*Â*Â*Â*Â*Â*Â*Â*Â*Â*Â*Â*Â*Â*Â*blue

-14.0 to -16.9Â*Â*Â*Â*Â*Â*Â*Â*Â*Â*Â*Â*Â*Â*Â*Â*Â*green

-17.0 to -19.0Â*Â*Â*Â*Â*Â*Â*Â*Â*Â*Â*Â*Â*Â*Â*Â*Â*black

The color coding scheme described above is only used with the model
8122 tubes manufactured by Burle Industries.