What does hj10k mean on my 1954 class ring?

The "HJ10K" marking on your 1954 class ring likely indicates the manufacturer and the metal quality. "HJ" could refer to the initials of the jeweler or company that made the ring, while "10K" signifies that the gold used in the ring is 10 karats, meaning it is composed of 41.7% pure gold. This marking is common on class rings, indicating both its authenticity and quality.