I can not find any table that tells me which wire IQI to use for single wall technique IQI film side either class A or B and also I cannot find any where it says that the film side IQI for single wall technique can not be used.
According to ISO 17636, For exposures in accordance with 7.1.6 and 7.1.7, the IQI type used can be placed either on the source or on the film side. If the IQIs cannot be placed in accordance with the above conditions, the IQIs are placed on the film side and the image quality shall be determined at least once from comparison exposure with one IQI placed at the source side and one at the film side under the same conditions.
I see your point. My understanding is that if you cannot use on source side, then, same IQI could be used on film side. But it sounds a bit strange. Lets see if anybody else have some suggestions. i am not that good in regards to RT.
As per ASME V – Art. 2, you can see T-275 and Table T-276 give you all info you need but ISO 17636-2 has this dark point, as you said maybe someone else has another interpretation for lighting the point.