I have a bottle of Royal Salute Chivas 21yr. What does the serial number on the bottle stamp tell me? ?

I picked this bottle up at an acution for $30. It came in the original box and by the looks of it, it seems to be pretty old.|||THat sounds like a good deal you got there...last bottle of Royal Salute I bought was around $120...although, I couldnt tell you how to decipher the serial number...sorry...