I'm not a teaware expert but I knew that these were not just cups that someone had picked up from a department store. A quick Google search revealed that after World War II, half of the Japanese exports to the United States were required to have this stamp. It was used from 1945 through April 25th, 1952. I found a very helpful porcelain collector site called Gotheborg.com that listed many of the maker's marks from this time period. I believe that my cups are from the Tashiro Shoten Ltd porcelain shop in Yokohama.
These are by far the oldest pieces in my collection. I don't think that I will be using them often due to their age and the fact that they are hand painted. Perhaps some day there will be a very special occasion where I will decide to pull them out. For now, I'm happy to just admire the tiny details of their design and contemplate how far they must have traveled all of these years.