What is a mess tin?

Answer 1

It's something Boy Scouts use.

A "mess kit" (US) or "mess tin" (British) is an aluminum clamshell. If you open it you have two plates and a set of silverware. You have to clean it in hot water after you use it if you don't want food poisoning.

You won't see this used in the field by soldiers for a few reasons. If you don't pad the silverware properly it'll rattle around in the mess kit and make noise that's detectable by the enemy. If you DO pad it properly you've probably wrapped your fork in a sock, which you're eventually going to need to put on your foot; when you do you've unpadded your fork and, once again, it'll rattle around and make noise. It's silver so it reflects light the enemy can see. (It's a dull silver but it's still visible.) The immersion heater you need to clean it produces a heat signature visible for miles away. It takes up space in your mess kit. And it's another thing to lose in the field. Troops who aren't eating MREs are dining on rations that come with paper plates and plastic silverware.

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A "mess" in military terms is a the dining room - where the soldiers/sailors eat. A mess tin is a metal (normally aluminium) dinner plate, with indentations to take different dishes - main meal, dessert and bread - which the soldiers eat off.