By enlarging the photo in post #1, we can see the tapered bellows and, I think, a black-painted flat panel between the column rails. That makes this a D5. The D6 is similar, but has no panel between the rails. The D6 was equipped with the fine-focus unit as standard equipment. I don’t see the fine-focus unit, so this is probably a D5. I can’t see enough of the machine in the photo to determine if it’s the shorter bench-mount model or the more-common D5XL. (XL= extra-long column).
The “Pro Lab 4 X 5” only refers to the condenser unit and lamp house in this case.
Here’s the D5XL with the Pro-Lab lamp house, condenser unit, and optional fine-focus unit (to the right and below the lens board).
http://www.khbphotografix.com/omega/Enlargers/D5.htm
The following link shows the D6. Note the lack of the stiffening panel between the rails and the counterbalance springs mounted on the top of the column. The fine-focus is standard on this model.
This article in the following link from KHB Photographix states that there were two versions of D6 and describes the differences. The one most often seen is the D6. The second version was labeled “Super D6 Pro Lab” and according to the article, is essentially a D5XL with the fine-focus as standard. Since it is based on the D5XL, I presume, but don’t know for certain, that it has the same rear-mounted counterbalance springs and stiffening panel between the column rails of the D5.
http://www.khbphotografix.com/omega/Enlargers/D6.htm
Due to the long bellows necessary to accommodate lenses up to 150 mm, it might be difficult of even impossible to use a lens as short as 50 mm for 35 mm enlarging with a D5 or D6. With my D5XL, even with the bellows fully collapsed into a flat stack (undesirable condition), a 50 mm lens is simply too far from the film to attain focus for a print of 8” x 10” or larger. The small-size lens-mount receiver makes the design of a recessed lens board for short focal length lenses impossible. I reserve this machine for lenses of 90 mm and longer for medium formats and 4” x 5” films.
I also have some Beseler 23CII enlargers that I use for 35 mm up to 6 x 9 cm films. Both the Omega D5XL and the Beseler 23CII are good machines. Each has its own strengths and weaknesses. You simply have to learn how to get the results you require. This starts by choosing the correct machine for the job at hand.