... with you in that collecting is very adicitive. But very pleasurable as well!
Regarding the difference among these tow references, I feel our friends here are more knowledgeable to explain this. Anyway, I think I can remember that they are basically similar, but the 5508 denomination came when Rolex decided to introduce the 5500 new series of numbers asigned to their references.
We´ll look forward from answers from Nicolas, Jeff and other members about this matter.
Enjoy your SC Sub in good health my friend!
Very cordially, Abel.