I looked for this and fortunately it wasn't difficult to find.

Around line 4442 (if you already changed the xslt file it might not be exactly that line) there's a line that reads:

<xsl:value-of select="b:RefOrder"/>

Change it to this:

<xsl:text>[</xsl:text>
<xsl:value-of select="b:RefOrder"/>
<xsl:text>]</xsl:text>

That should have the desired effect.