Differences in CIS and non-CIS gas tanks
 

I bought what I believe is a new (reproduction) gas tank for my 73.5 CIS car from a North Georgia Porsche parts place last year.
Lately though I been seeing in a few parts catalogs pretty much the same tank, but with a claim, "not for use for CIS cars". Hmmmmm.

The tank I bought looks the same with the large space for a full sized spare tire, but not knowing what is inside that makes it strictly CIS is a mystery to me. Without jacking up my car for an inspection, just how can I tell a CIS from a non-CIS tank, especially from the top view they appear the same?

Thanks
Bob
73.5T