Mailmerge is probably going to get in your way on this one, unless you
simplify the process a bit. e.g.:
a. create the letter as a non-mail merge document
b. set up a separate mail merge main document as a single table row (and
nothing else except the final paragraph mark, which you can't remove) with

Column A
{ MERGEFIELD First_Name } {MERGEFIELD Middle_Initial } {MERGEFIELD
Last_Name }
Column B
{ NEXT }{ MERGEFIELD First_Name } {MERGEFIELD Middle_Initial } {MERGEFIELD
Last_Name }

and use the Select recipients dialog box (or Merge Options, depending on the
version of Word) to specify Anniv = 5.

c. do the merge to an output document
d. use Edit Copy/Paste (or save the output document and use Insert|File) to
insert the output into the document you created in (a).

Or you could consider creating the Excel sheet you need and copying/pasting
it directly into the document you created in (a).

If you are producing one document for each of a number of recipients
automating that approach would probably be the simplest way to do this,
although there are alternatives.