I'm not sure how it is working without the ID tag? 
I assumed that you would just put the index bar at the top or bottom of your category pages so it would automatically get passed in. I think that you will need to feed it in when you link to the page from your category page:
<a href="<%db_cgi_url%>/page.cgi?p=cat_index&ID=<%ID%>">Index</a>
or straight onto letter A
<a href="<%db_cgi_url%>/page.cgi?p=cat_index&ID=<%ID%>&letter=A">A</a>
I assumed that you would just put the index bar at the top or bottom of your category pages so it would automatically get passed in. I think that you will need to feed it in when you link to the page from your category page:
<a href="<%db_cgi_url%>/page.cgi?p=cat_index&ID=<%ID%>">Index</a>
or straight onto letter A
<a href="<%db_cgi_url%>/page.cgi?p=cat_index&ID=<%ID%>&letter=A">A</a>
