Differences in Case Design

I noticed something on my 3rd member which looks different than others I've seen. The normal style is picutred below. The front has a waffle appearance with gaps between the raised grid pattern on the case.



While mine has the same grid pattern, one of the holes is filled in. I've not seen one like this before. It does not look as if it was filled by welding. It looks more as if it were cast that way but I don't know why. The back (inside) of the case at this spot looks perfect.

I've made a few postings to figure out the reason. No one has seen one with this feature. Additionally, the part number, date code, and other items are different. I did not notice but it was pointed out by Jeff Speegle. It is believed this was just sourced from a different (and not often seen) casting plant or subcontractor.

I learned the DIF casting stands for 'Detroit Iron Foundry'. Many believe it is an abbreviation for differential.