Alternatively, if it's an Excel data source, you could use a another column with formula to extract those letters, then reference that column in the mailmerge. If you put the field that holds that result somewhere in the body and give it the same colour as the background (and even make it as small as 1pt) with an appropriate character Style, it won't show up in the body but can still be referenced via a StyleRef field in the header.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]