NB!! It is the Eg-S (6D version) that works on the 5DIII, not Ec-S!
Ahhh!! that explains why I couldn't force my Ec-s into the 5DMK III cavity.
Will order the Eg-s now & try that.
Sorry I was a bit unclear on that to begin with. I hope you didn't break anything ...
1DXII, 5DIV, 5DSR, 8-15/4L, 11-24/4L, 16-35/2.8L III, 24-70/28L II, 70-200/2.8L IS II, 100-400/4.5-5.6L IS II, 200-400/4L IS 1.4x, 17/3.5L TS-E, 24/4L TS-E, 35/1.4L II, 100/2.8L IS Macro, 300/2.8L IS II, 600/4L IS II, Zeiss: 15/2.8, 35/1.4, 55/1.4 Otus, 85/1.4 Otus, Profoto B1