clip away the bits you don't want to show e.g. to show the bottom half you can do this...
<defs>
<clipPath id="clip1" clipPathUnits="objectBoundingBox">
<rect y="0.5" width="1" height="0.5" fill="black" />
</clipPath>
<symbol id="MySymbol" >
<line id="L_010" x1="7" y1="20" x2="23" y2="20" style="stroke:navy; stroke-width:2" />
<line id="L_020" x1="7" y1="40" x2="23" y2="40" style="stroke:navy; stroke-width:2" />
<line id="L_030" x1="7" y1="60" x2="23" y2="60" style="stroke:navy; stroke-width:2" />
<rect id="R_00" x="10" y="0" width="10" height="200" style="fill:#A0B0C0; stroke:navy; stroke-width:1" />
<line id="L_000" x1="0" y1="1" x2="30" y2="1" style="stroke:navy; stroke-width:2" />
</symbol>
</defs>
<use x="400" y="10" width="30" height="200" xlink:href="#MySymbol" clip-path="url(#clip1)" />