Look at his legs,glute to ham tie in etc etc (oh shit iv been here to long )
Glutes and hams are perhaps not the best indicator of conditioning. Hell, there are current competitors that get extremely lean, that are smooth in those areas. 8% is not really difficult to achieve. I wouldn't suggest Arnold as having been as lean as today's standards, but he most certainly dipped below 8%
at his very leanest. He has cross striations in his chest, excellent quad separations and a Christmas tree on his lower back.
During earlier competitive years, he wasn't as lean, but in 1975 specifically, I would respectfully suggest his body fat was at least a little South of 8%.