Hi Brian & Alf,
The III Bomber Comand of the Third AF and the 56th CCTW of the First AF (which was transferred from the Third AF) used a Base Unit field buzz number system: X X#, first letter is the field, then a space, then a squadron assigned A/C letter and then a number assigned to the BU squadron.
The pre BU field buzz number system for III Bomber Comand was blocks of numbers in the thousands for the unit and plane number in that unit.
Attach are examples of B-26s at Lake Charles pre and post BU reorganization.
Yes, they did use the confusing letters I and O for the assigned A/C letter in the BU system.
Hope this helps you.