Each and every individual mandolin/guitar/banjo part which was used to build an instrument was not marked with a FON. As far as I know, in a completed pre-war Gibson mandolin the FON appears in only one place, either on the interior sidewall, or on the interior of the back.
When a batch of instruments was completed all the material used to build them would have been charged off to the FON in accounting at that time. However, any remaining, extra parts were simply stored. These extra parts were not marked with the FON under which they were originally built. So they clearly appear anonymously in later batches.