terrier,

The newer canteen cups should be aluminum. They have been that way for a while (at least 8 years) as they are lighter than stainless steel, but I have been known to be wrong

As far as the letters in the middle, they are lot numbers. The numbers may be a year or what ever, and the letters are the manufacturer and the lot designation. I looked at mine and I gave: US PAC FAB. I am sure that is the Manufacturer. Not all US military gear is made by one company as you know, so each company gets to stamp their info into the cup. To tell you the truth, I rarely use mine at all in the field. I keep it in my canteen pouch to keep it open when I pull out my canteen and put it back in.

Garrett
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On occasion of every accident that befalls you, remember to turn to yourself and inquire what power you have for turning it to use. - Epictetus