Use a flashlight. It is probably stamped in black ink on the inside under the rim strip, If you shine the flashlight along the surface you should be able to find it. A camera flash brings it out as well once you find it. (Take a pic of it to read it).
The earlier ones were always stamped in plain english in black ink on the rim. Its just that the black rims are hard to see it. I'm not positive those later models were still stamped, but my guess is that they are.