According to the Ordinance Manuals of that time period, tin was tinned iron sheets. Hopefully the person who made it used lead-free solder. In my humble opinion the vendor that sold you a rusty cup is a bit questionable to say the least. If it was so poorly made as to be rusty, did he acquire the cup from a third-world country that still uses lead solder? Always purchase from a vendor with a good reputation and a vendor where you know where the cup/item was made. If you don't know where or who made it, ....personally I would not risk my health. Also, you can remove the loose rust with a 3M pad, either one from a big box store's paint department or a heavy-duty type dish sponge from the grocery store. The key words here are "loose rust", for a patina finish you leave a hint of oxidation on the item. After you remove the loose rust with the 3M pad, apply a food-quality vegetable oil if you are going to store the cup for a while. Food grade mineral oil from a grocery store is different from mineral oil from a hardware/garden store. Don't use mineral oil, use vegetable oil, it is what the blacksmiths at the reenactments are using as a food-safe finish. The vegetable oil creates a patina, more or less like the finish on iron pans that have been "seasoned". Seasoned iron pots and pans basically have a patina finish that is a light coating of oxidation(rust) that has vegetable oil baked into the oxidation.