Thanks for everyone's help. Any direction that you can give me is very much appreciated.
There are no visible marks on the cups anywhere. They feel like and tarnish like silver. I cleaned one up to take pictures of it.
The rest of the back story is that my grandfather was president of a regional grocery chain in the 50s and 60s. At some point, a beer distributor, he remembers Budweiser, gave these cups to him as a gift. They were going to throw them away and I thought they were too unusual to pass up.