ricardian's algorithm certainly wasn't the one used in the 1990s. With that algorithm, the letter will advance by 5 every time the number goes up by one, and by 14 every time the tens digit ticks over (e.g. xxxxx59 to xxxxx60). But in numbers from my era, the letter advances by 1 per number, or 4 when the tens digit ticks over.
2....58J
2....59K
2....60P
2....61Q
etc.