Nitrogen Small military cylinder
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Small "military" cylinder.
These cylinders look like the little CO2 cylinders used in BB guns. A reader, Christoph Loew, reports:

"The cylinders of nitrogen gas are, as far as I know, used for cooling the thermal imaging system of certain man-portable anti-tank missiles, I did some maintenance work on thermal imagers during my time with the German army and was told that the MILAN system used these."

Further more, reader Michael Z. Williamson forwards this from a buddy:
Howdy Mike
Yup, just what they say they are. We used same or larger ones for Dragon and TOW system, Depending of system fittings, before the first chillers powered off batteries or vehicle were deployed on the AN-TAS-4A Thermal for the TOW. I never handled the babies as we used the updated system only for training but the manuals did list them as accessory for the earlier version. They may also fit the Sight purging kit for several systems. They are a NATO standard cylinder.

Sarge the Poet
Joseph A. Merrill III

So now you know.

Source: eBay seller janerili
Contributor: Theodore Gray
Acquired: 5 September, 2002
Text Updated: 20 November, 2008
Price: $5
Size: 2.5"
Purity: >95%