That wart looks to be for additional vacuum port, say vacuum guage or something but ye probly knew this already. Tom, if it was me I'd be worried about polished shiney valve cover not dispersing heat and then warping as they're sometimes known to do. Looks be leaking a little already on left rear corner. Valve covers are easily machined to restore flat surface. But strikes me polishing em aint a good idea. Yer 108/617 5-spd looks to be the ultimate diesel MB!! Heck I'd swap my 116 300SD for it any day of the week.