Phil Davis' "Beyond the Zone System" also confirms that the ISO/ANSI standard uses 0.1 over b+f (when developed to a CI around 0.6) as the speed point.(posted 8174 days ago)I checked my TMZ box and indeed, it is an EI 100 film, not an ISO 100 film! On the film cassete (35mm) it just says 100/21 and doesn't mention EI or ISO. On the other hand, Tri-X is ISO 400 and FP4+ and HP5+ are also ISO, as is Acros.
I'm just glad I don't need to shoot my Sensia 100 at EI 50 too! At least slide films are pretty much "as advertised". Of course they use a different standard for film speed too.