2K789 is a 3.5 inch mounting.

https://www.amazon.com/Dell-Optiplex-Metal-Drive-Caddy/dp/B019JCOJ9C

If you don't want to use R494D then buy a silverstone adapter.

https://www.amazon.com/SilverStone-Technology-3-5-Inch-Converter-SDP08-LITE/dp/B00EHDZLDM/

 

 

If the metal cage is equivalent to the standard blue plastic one I wonder what the reason is for choosing it. The blue one is easy to use and does not require any tools. It would not seem that durability is much of a requirement but perhaps the metal one would withstand more use if someone is swapping out drives more often. As for the Silverstone 2.5" adapter, reading some other comments online prior to posting this original question it looked like the alignment of the 2.5" within the cage must match the offset of the Dell setup due to the tab that blocks one side of the blue cage. If the R494D is the official part I am happy to use that, especially since it holds two drives.

Does the Dell R494D get snapped into the standard blue 3.5" hdd cage functioning as an adapter, or is it a standalone replacement for the standard cage that slides directly into the 3.5" bay in the case?

And it takes about 4 hands to do it.  Leave it up to engineers to make it difficult.