Here is what I found for the 24 cards in Series #6 so far. It looks like the first print run has no letter. Those can be found printed either in Japan or Italy. In addition, I am finding print run "A", "B" and "C". So I'm working on getting all five versions for Series #6.
I have every card from the blank print run (Japan), "A" and from "C". I am only missing one card from the "B" print run - Olympic Hierarchy (06-19). I only have six cards from the blank print run from Italy.
Here are the back variations I've seen. Without really checking I found one variation on the front of the cards.
6-02 Indianapolis (Auto Racing) - 'A' and 'B' have 1978 results; 'C' has 1979 results
6-09 Paula Sperber - Name on Front and back; in print runs blank (Japan), 'A' and 'B'
6-09 Patty Costello - Name on Front and back; Only in print run 'C'
6-10 Gold World Cup (Nicklaus/Miller) - 'A' has 1977 stats; 'B' has 1978 stats; 'C' has 1979 stats --- 4 variations
6-15 Aztec Stadium - blank has old caption; 'A', 'B' and 'C' mention soccer
6-20 Alpine World Cup - 'A' has 1977-78 stats; 'B' has 1978-79 stats; 'C' has 1979-80 stats --- 4 variations