From KHB Photografix:
"Note: Early D-6 enlargers used an early version of the turret (Cat. #404-866) with a thicker mounting flange. The two types of turrets look alike but are not interchangeable. If your enlarger is an early D-6, we can substitute the early turret on request."
Also take a look at this KHB Photografix Omega D5/D6 Lens Mount Reference Chart.