No, not all rings are stamped. Most eternity bands and rings that have diamonds that go all the way around the band are not stamped because the holes for the diamonds are in the way.
Its the jewelers mark, its the initals and trademarks of the jeweler (and/or company) that manufactured it. It may also contain the karat value of the diamond(s) and gold.
If you look closer, you will likely see that it says 925, rather than a25. This refers to the purity of the silver, (92.5% silver, otherwise known as sterling).