I had wondered how Sinnott managed to handle the work load of Marvel ink jobs, pencilling for Charlton, and doing outside projects. One possible answer is his did super-simple, repetitive Charlton layouts. Lots of head-and-shoulder stuff. I'm sure he just roughed them in knowing that Colletta would finish them in his own style anyway. Sometimes if you look really hard you can see one of Sinnott's faces, usually on a middle-aged man. Colletta had his good points, but his domination of the Charlton romance line was a big part of their dreary sameness.
Link to the book:
Sue and Sally Smith, Flying Nurses 51