1st cent Patera

[jvrjenivs]

The reason for tinning is simple. To cook your food in a brass patera is toxic. You need lead-free tinning on the inside to be food-safe, although silverplating will work as well. (or get a patera out of 325 sterling silver, of course).
Next would be the simple fact that most people use a patera made by Deepeeka, e.g. just one sample.

As for your size and look question I would simply say that there isn't a militairy style. Patera were found accross fortresses with different look, sizes, etc. Just don't take highly decorated, silver version for a soldiers marching pack. A bronze one will be perfect, although I would still have tinning on the inside.