By the way, I pulled up a copy of the 2004 GCR. The rules for IT are effectively the same, but referred the reader to TWO sections of the GCR, 17.12 and 19.

17.12 was pretty much the same as the current one and had that same verbiage, "All cars shall be equipped with a fuel cell complying with specifications according to GCR Section 19., except Showroom Stock, Touring and Improved Touring cars."

19, however, was a complete discrete section of the GCR detailing how a fuel cell was to be constructed and installed. It started with:
All safety fuel cells shall be constructed and certified in accordance with the FIA FT-3 specifications. Where safety fuel cells are required in SCCA Competition, all safety fuel cells shall consist of a foam-filled fuel bladder enclosed in a metal container at minimum.
In order to comply with the ITCS you had to comply with GCR 19, and while there were no expressed exclusions for IT cars, there are still those "where safety fuel cells are required" weasel words...

I still can't imagine SCCA allowing a safety item in a car and not requiring it to meet a spec, but it's certainly vague enough to make one go ...