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When your 203FE is modified to be used with the CFV BACK and F(FE) lenses,
then the ISO setting on an E12 (E24) magazine is not transferred to the camera any more . That was a restriction , which results from the modification . The standard setting then is 100 .
For that case , you would set the ISO of the film you want to use , manually in the camera and then it does not matter if you use A-type or E-type magazines .
If your 203FE is not modified for the CFV BACK usage , you can use the E-type magazines with their ISO settings (according to the film you use) , but also any other magazine , for which you then have to set the ISO manually in the camera as well .