[xsl] selecting specific elements from a second file

["Mark Wilson mark@xxxxxxxxxxxx" <xsl-list-service@xxxxxxxxxxxxxxxxxxxxxx>]

I want to select elements from the second file to add to the first file 
when their @pofis-number match (in this instance 2966 has a match, 2967 
does not.
Any help appreciated.
Mark
-----
File to Merge to (simplified: the element <Stamp> is much further from 
the root)
<List>
.....
    <Stamp>
          <CatNumbers scott-number="2816" pofis-number="2966"/>
           <Value kc-value="3"/>
           <Inscriptions czech="DS. skauting, 1911-1991" 
eng="Czechoslovak Scouting, 1911-1991"/>
           <Inscriptions czech="A. B. SvojsC-k, 1876-1938" eng="*"/>
           <Location denomination="-6"/>
     </Stamp>
      <Stamp>
          <CatNumbers scott-number="2815" pofis-number="2967"/>
           <Value kc-value="1"/>
           <Inscriptions czech="MistrovstvC- evropy v judo, Praha 1991"
                       eng="European Judo Championships, Prague 1991"/>
           <Location denomination="-6"/>
     </Stamp>
.....
</List>
-------
File to merge from:
<List>
   <Flaws pofis-number="2966">
      <Known plate="2" position="37" file-name="2966-DV37-2.jpg"/>
      <Known plate="2" position="38" file-name="2966-DV38-2.jpg"/>
     <Unknown number="1" file-name="2966-mn1.jpg"/>
   </Flaws>
</List>
------
Merged file:
<List>
    <Stamp>
          <CatNumbers scott-number="2816" pofis-number="2966"/>
           <Value kc-value="3"/>
           <Inscriptions czech="DS. skauting, 1911-1991" 
eng="Czechoslovak Scouting, 1911-1991"/>
           <Inscriptions czech="A. B. SvojsC-k, 1876-1938" eng="*"/>
           <Location denomination="-6"/>
           <Known plate="2" position="37" file-name="2966-DV37-2.jpg"/>
           <Known plate="2" position="38" file-name="2966-DV38-2.jpg"/>
           <Unknown number="1" file-name="2966-mn1.jpg"/>
     </Stamp>
      <Stamp>
          <CatNumbers scott-number="2815" pofis-number="2967"/>
           <Value kc-value="1"/>
           <Inscriptions czech="MistrovstvC- evropy v judo, Praha 1991"
                       eng="European Judo Championships, Prague 1991"/>
           <Location denomination="-6"/>
     </Stamp>
</List>