Rick Rothstein was thinking very hard :
"=MATCH(LEFT(RC2,LEN(RC2)-8),C1,0)"

The OP said "if their left 16 characters do not match..." so why LEN-8
instead of 16?

Rick Rothstein (MVP - Excel)

Because the data to test is 8 characters longer than 16. Like I said,
if all values are 'Text' then it doesn't matter but if the short values
were formatted with leading zeros then we have a problem as occurred
when I tried your solution on formatted values. (Ihad to adjust for
shorter length to avoid scientific notation from occurring)

Hey, if Gary's data is exactly as posted (Text) then all is good!

--
Garry

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