I think the assumption that sail numbers are assigned from 1 without gaps is one that's been violated in the history of the DS. That would be the explanation for your sail number.
Also, while the number does not contain any letters, you would have the DS logo (the D with the dropped S intertwined) above the numbers.
One set of numbers on either side of the sail, with the starboard side placed higher. So the order from the top is starboard logo, port logo, starboard number, port number.
That said, you only need the number if you intend to race in a DS Fleet or other event where they record sail numbers. The DS logo is another matter - it's a nice conversation starter "what kind of boat is this?".
Your old sails may have had the wrong number (unless they were the original set). Many people buy used racing sails for cruising and may not always bother to remove the (wrong) number.
My boat came with used sails like that (but with the number removed) and now has new sails, still w/o number, as I didn't have the handy hull number plate like you do and it took a lot of detective work to arrive at a likely number (which I then formally reserved with the DSA secretary - just in case I ever need it).
The weekly beer-can race I participate in couldn't care less whether my boat has any number in the sail, heck, they didn't even blink when I told them it has no name. Until the day I'll stumble into one of the first three spots, they won't even record I was there

Because the race is not one design, that would be an unlikely event - there are usually more than three other boats there that are much faster by design and sail area than the DS.
