For the 1943 ORER, Ethyl and Hooker list TMI for their ICC105 cars (two of the owners who do list the ICC type.)
TPI wasn't included in the tank car type table in editorial section of the '43 ORER, and the ARA/ICC cross reference shows TM for type 105 (Footnote says to add I to the tank car class if insulated, so that would make it TMI for type 105.)
When was TPI started by ARA?
I guess I could cross reference a later ORER that used the TPI designation? In the '43 ORER you need to peruse the notes, and they appear to be incomplete for some car owners.
The '36 freight tarriff 300-A tank car book uses notes that identify the commodity shipped for specialized cars. I guess that was the way to find a chlorine car - look for cars with note 10? For a modeler, that may actually be more useful than just ICC105 (since chlorine cars were almost all ICC105, except for the cars with racks of small tanks?)
Is it even possible to find WWII era tarriff 300-A's?