Gotcha on the 951. I should have known.

I got a 12pt/double hex bit for my 603... or is that 961 ... at Pep Boys. Fits into a 13mm socket and is just long enough to work on the recessed 603 head bolts. I think it was part of a set of 4 for less than some places want for a dedicated bit.

Many other cars use this style of head bolt drive.

Sixto
91 300SE
87 300SDL
83 300SD