From Bill Gunston’s ‘The development of jet and turbine aero engines’…

In Rolls Royce case while Griffiths rightly kept thinking of BPR around 20, a study was made to find optimum BPR for an engine in an external pod and, because of incorrect assumptions of nacelle drag, came up with the answer that it should not exceed 1.0.’

Anyone know when this study was done?

I assume you mean by-pass ratio, but I’m not sure what you are trying to point out.
One thing I can tell you is that a high BPR does not neccessarily mean a better propulsive efficiency. The BPR can be varied depending on a/c range, number of engines, a/c mach number, even noise emission.