Some first principles
For the B737 I have nothing to hand.
The B767-300ER has, by the best estimates that I have seen a zero-lift drag coefficient in the range 0.0130-0.0135 and an Oswald wing efficiency factor of 0.835.
Its wing area is 3050sq ft and the unfactored aspect ratio is 7.98. Taking account of the Oswald coefficient the factored aspect ratio is 6.67. Thus induced drag coefficient is CL^2/20.95
At speeds for which compressibility can be discounted the best L/D ratio to be expected is when zero-lift drag and induced drag are to all intents and purposes equal.
For this to be the case CL = .53 and zero-lift drag coefficient is 0.00134. Suppose the weight of the aircraft is 310000lbf then under ISA conditions at 33000ft pressure altitude and M0.708 we have the necessary conditions and can assume compressibility drag to be vanishingly small. L/D = 19.8 and the airspeed is 238keas.
Bearing in mind the definition of equivalent airspeed and also the roughness of the foregoing assumptions I wonder if Boeing intended the best glide speed of the B767-300ER to be 240keas with an L/D of 20 and therefore a glide slope of 1 in 20. Just a thought.