I wonder where the body would have been supported during the application of the under coating at the factory?

I also highly doubt that dirt and grime will leave an almost perfect shape like that. I still think it is factory under coating and a cardboard template was used or something was blocking out that area for a reason. If it was grime or dirt, I would imagine that it would come off fairly easily with digging my finger nail into it unlike the undercoating would. But again, I am no expert on this. I am just going by observation and taking into account the other areas that were taped off or covered up while the undercoating was applied, similar to the areas where the leaf springs mounted up to.