Upon disassembling my transmissions, I have come across an oddity. The nuts that hold the differential cover on all have little helicoils in them. Why?

Multiple transmissions have this, so it wasn't just some guy who had a pile of metric helicoils but couldn't find any metric nuts

yep...the 6mm nuts on the back of the /4 oil pump have the same thing....no clue why they did it. If the heli-coils are stainless they would be harder, but why not just make the whole nut harder.

I thought of a semi-plausible reason! The studs need to hold well, but are in magnesium... if they are cast into the case instead of threaded, with some sort of square head under the surface, you would NOT want to turn the stud. The helicoiled nuts would give another place for the nut to break loose without turning the stud, if the nut ever got stuck.

Nope...

I take the studs out of VW cases all the time, I have to put in longer studs for a mount bar I use on all hipo customers tranny's. The VW (bug) and 914 tranny are built basiclly the same way...just the shape, gears and etc are different.