Aggie: Undiluted D-76 contains a high concentration of sodium sulfite which is a silver solvent. Thus film developed in undiluted D-76 should show slightly finer grain. Freshly mixed D-76 shows a higher level of activity than aged D-76.
"D-76 may be used undiluted, but there is no advantage in doing so. The negatives, while slightly finer grained, do not exhibit the same degree of sharpness or tonal scale."