Hobie sail numbers are not assigned to the boat like other one-design classes. For instance, in Flying Scots you register with the class association and are then assigned a sail number. Whenever you get sails you're supposed to use your class assigned number. Lots of one-design classes work this way. Hobie sail numbers are sequential, but apparently sequential in the sail making process only. You might be able to figure out what year your sail was made (if you're lucky) but that will only mean anything about the boat if you are certain it is the original sail, or if the sail was replaced by someone who requested the original sail number be put on it. For instance, when I bought my '79 Hobie-18 in about '93 (man, I need a new boat!) it had a sail number of 14863, on newish sails. I'm pretty darn sure that was not the original number for an 18 that old. I recently bought a new sail from Hobie for it, wasn't even asked about sail # preference (I was figuring I'd be putting the numbers on) and it came through nicely numbered 16744. My guess is that whenever they make a main they just put the next number on it and ship it. One item: The sail is very nice, well made, and has enabled me to kick some butt in the local Fleet - if you're feeling sluggish, try new rags from Hobie.
So here's another serial number and year of manufacture thing - if you see an "R" after the number, it means that you have a replacement hull. Both of my hulls have "R" on them, so even though the number indicates it's a 1979, I have no real idea as to when the hulls were actually built or why they were replaced! I've always figured I could call Hobie and get some answers, but it sounds like that might not be the case if they don't know which end of a boat is which!