Hi Richard,
We have always truncated to the last 4 digits or occasionally been able to get away with three. It is written into the SIs that we can change a sail number by adding a digit if two 4-digit numbers clash.
We also have a four 4 (or three) digit column. We only need to find competitors visually on the lap sheets, this is sped up by instantly knowing the class e.g. a topper is likely to only appear once whereas a 600, three times. The usual issue is digits reversed or mis-read in which case the Wizard is of limited use.
Things happen in club races that shouldn’t happen in an Open: no sail numbers, different sail numbers on each side of a new sail, a family of Aero Sailors out with 3 sizes of sail all with the same sail number. We have signing on sheets with columns for ‘sail used today’ and ‘series sail number’ as in the real-world competitors change boats and have old sails for windy weather (And scorers have limited patience in sorting it out).