I came across this picture of a North Korean overfolder stock somewhere and saved it in my "AK pictures" directory. I've also seen pictures of two or three homebuilt rifles with PPsh43 overfolder stocks.
I kept trying to figure out why anyone would bother, until I realized a sidefolder would foul a drum, and so would an underfolder, of course. If you wanted to be able to use a drum and wanted a folding stock, overfolder makes sense.
The folding stocks let you pack more rifles into less space, good for shipping, motorized infantry, tank corps, etc. If you want lots of firepower, you just swap your 30 or 20 round magazine for a drum.