LJ Pt950 A may be the steel seal of jewelry merchants, in which "LJ" may be the abbreviation or synonym for jewelry merchants, and Pt950 stands for platinum with purity of 95%, which is also a common jewelry material. A may indicate that the metal part of the jewelry is Grade A, but if the jewelry is bought in a gold shop, A indicates that the metal part of the jewelry is gold.

As for what you said, there is a square outside LJ and a circle outside A, I don't quite understand this, because it is not some general signs or rules. It may be the logo of some specific businesses or factories. It is suggested that you consult the merchants or factories directly, and they can give you more accurate answers.