The Omega D5/D6 has long tapered bellows to accommodate 150mm lenses. That coupled with the small receiver for the slide-in lens plates makes it impossible to bring a 50mm lens close enough to focus an image large enough to make a borderless print larger than about 8” x 10” (some lenses can just barley make 11” x 14” due to a slightly longer flange-to-film distance ). Because the bellows are fully compressed the lens is simply too far from the negative.
The D5/D6 doesn’t have room to accept a recessed board to place a 50mm or shorter lens close enough to the negative to focus prints larger than about 8” x 10” or slightly larger with a reasonable amount of cropping ability.
It doesn’t matter whether the lens is mounted on a turret or is mounted onto a lens plate secured to the slide-in plate. Most 50mm lenses can be used on a D5 or a D6 to make an 8” x 10” print. Some might just barely make 11” x 14”, but with no cropping ability.
Whether using the correct turret or not, the essential problem remains: a 50mm or shorter lens is always too far from the film to focus beyond 8” x 10” or, in some cases 11” x 14”.
The D5 and D6 need a longer lens than 50mm in order to focus an image at any magnification allowed by the available negative-to-print distance and the minimum bellows when fully compressed.
You can use a longer lens than 50mm for enlarging 35mm films on a D5 or D6 or you choose a different enlarger that doesn’t have this design constraint.