What is the meaning of 'ca 925' engraved on the inside of a ring?
It means it is pure silver.
925 indicates the purity of the silver out of a possible 1000. A purity of 925 is fairly standard because they take Fine silver (999) and add a small amount of copper to give it strength and beauty. 925 is more commonly known as "Sterling Silver". A US size 7 ring would be size N½ in Australia, Ireland and the UK.
925 is a standard stamp for sterling silver. Other marks include the word Sterling and abbreviations of that. The 925 show how much silver is in the metal, or 92 1/2% silver (with the rest being copper). That is the recognized and legal meaning of sterling jewelry metal. http://en.allexperts.com/q/Jewelry-Gems-Minerals-689/ring-made.htm