Another possibility- you didn't mention your serial #. The last 52 Mangustas were finish assembled under contract in south Germany by Armand Fischer, as DeTomaso was very busy with setting up for Pantera production at that time. So cars after about 8MA01134 came new with some over-the-counter Mercedes parts adapted- the handbrake, etc.
Minor scandal at the time: when Mercedes found out, they threatened Fischer with a lawsuit! No real way to tell what those late cars came with "stock," as some were 'more assembled' than others from DeTomaso, who really wasn't into keeping good records and Fischer is now also gone.