<boxes content="Grapes and Apples">
    <box No.="Box MT. 53">
      <quantity f="4">Grapes A</quantity>
      <quantity f="8">Apples B</quantity>
    </box>
    <box No.="Box MJ 62">
      <quantity f="7">Grapes A</quantity>
      <quantity f="12">Apples B</quantity>
    </box>
  </boxes>

<some text...>
<some text...>        
        <f><v>Begin</v></f>
        <f><v>Prod No</v></f>
        <f><v>Serial</v></f>
        <f><v>Grapes and Apples</v></f>
        <f><v>Begin 1</v></f>
        <f><v>Box MT. 53</v></f>
        <f><v>XMT. 5563</v></f>
        <f><v>Begin 2</v></f>
        <f><v>Box MJ 62</v></f>
        <f><v>JJKD. 772</v></f>
        <f><v>Apples B</v></f>
        <f><v>Grapes A</v></f>
</abc>

# I've added or modified a little bit your code (in blue) in order that be able to handle strings with spaces. 
#(E.g. instead of "Apples" and "Grapes" the string could be "Apples XXX YYY" or "Grapes abc" etc)

awk 'NR==FNR {
  $0=gensub(/(.+=")([0-9]+)(">)(.+)(<\/.+)/, "\\2 \\4", "g");
  if($1 ~ /^[0-9]/){ t=$1; gsub(/^[0-9]+[ ]+/,""); a[$0]=a[$0]" "t }
  next
}
{
  gsub(/.+<.>|<\/.+$/,"")
  x++;  if($0 in a){ b[$0,1]=x; }
}
END{ j=2;
  for(i in a) {
    al=65; split(a[i],arr," ")
    print "<begin ln=\""j"\" >"
    printf("\t<c ln=\"%c%d\" t=\"s\"><v>"b[i,1]"</v></b>\n",al++,j)
    for(v in arr) {
      printf("\t<c ln=\"%c%d\"><v>"arr[v]"</v></b>\n",al++,j)
    } print "</begin>";j++
  }
}' file1 file2