Could I make a Nelson EOG desk from a credenza?  

Credenza
Here's a credenza by itself. Looks pretty similar. I'm wondering if the drawers, etc. are any different.

Anybody have any input here?
Anybody have any input here?

If you separate
the two pieces you will have two big holes on the left of your credenza.

Holes
So the actual EOG desk is connected to the return by more than just sitting on top? There were no holes on the one I looked at, but it may have been because it was an early model.

Yes
There is a mechanical connection between the desk top and the return/ credenza (you are right, they are basically the same thing). You can see it in the photos you posted. There is an L shaped piece that makes the desk top "float" off the return and there are 3 or 4 long bolts that connect the desk top to the return from underneath.
So yes, you could buy a credenza, have a desk top fabricated, and attach the two to make your own EOG. You could have the modesty panel recreated as well I suppose. The harder thing would be to have a set of matching legs made for your desk top. Difficult, but not impossible.
The EOG came with options such as the orientation of the return and the configuration of the return/ credenza. But if you find a credenza that is a configuration that would work for you, the desk top is probably the simplest part to make.

Thanks
Thanks Pegboard Modern. I'll go ahead and start looking for one.

I've always loved the term "modesty panel"
From Wikipedia:
"Early modesty boards were often used in Quaker meeting houses and other churches of the eighteenth and nineteenth centuries, growing in popularity during the Victorian era. Modesty boards might be found running up the sides of stairways to discourage passers-by from looking up females' dresses. They were also added to the sides of church organs, which served to shield the musician's feet and legs from the congregation's view as she played the organ's pedals.
Later, as women began to enter the secretarial force in large numbers during the twentieth century, modesty boards were added to office desks."

I agree. I've also heard...
I agree. I've also heard them called privacy screens. I like them for the way they act as a room divider as well.

If you have
A credenza and are going to use it as a return, I wouldn't put holes in the top to attach the desk to it as the originals were. You would significantly reduce its value.

Couldn't the desk just rest atop the credenza?
I don't see why it NEEDS to be attached in any permanent way. (The modesty panel's a different story, assuming you want want one- )

Because of the teeter tooter effect.
If you don't want to permanently affix the table top, then make sure to move the position of the legs more to the edge.
The way it looks originally, enough leverage is there for the teeter totter effect to happen, which is why the other end is bolted down.
Move the legs out to neutralize it, or be conscious of where you place your computer/books/ass etc.