As I have said in another thread, I am in the process of a timing belt change in a Golf BKD engine. When I bought the stretch bolts there are 2 that have a special head. It's a combination of external hex. head & internal XZN/multispline/triple square. These 2 bolts secure an aluminium block to the end of the engine block. The frame mounted engine mount is then fixed to the top of this block. Because the screws are difficult to undo & then to tighten I think these special heads have been tried. The originals are hex. head with a flange.
I have never come across these before.