Many boats use their series hull number as their sail number (ours does), so I assumed that must be what the O.P. meant.
But I'd agree with Paul, I've seen it done all sorts of ways. I'm not sure there is a single convention -- in the U.S. anyway.
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Pacific Seacraft Crealock 31 #62
NEVER CALLS CRUISINGDAD BACK....CAN"T TAKE THE ACCENT