Dell Keyboards

Optiplex units come with basic wired USB keyboards

Previously the ones with large/tall keys, and an outer keyboard shaped that fit around the bump downward of the space bar was:SK-8115.
     An additional part number for this keyboard (or one just like it) seen is: RT7D50

Probably the preferred one of the thin variety:
The thinner keys, and a rectangular shape to the entire keyboard is: KB-212B.
For some reason at the Dell site, the KB212 (without B) is $5 less expensive.

Also saw a KB1421, which has fairly tall keys, and the entire keyboard is rectangular.

Very thin and flat keys, and least preferred, though can type "ok" on it
 is KB113P

The Dell keyboard (with med/tall keys) and a volume knob on upper right is: SK-8135

7-8/2016 see a thin-key keyboard with some light blue on certain areas, KB216t coming with Optiplexes.