Design One
Chapter 1, "The Enemy Within"

Elliptical housing.
Nine circular holes.
Pattern-two, two in gray rectangule, one in center, two in gray rectangule, and two.
Seen on USS Enterprise.

Design Two
Chapter 5, "The Enemy Within"

Elliptical housing.
Pattern-large black rectangule, upside down trapezoid, large black rectangule.
Seen on USS Enterprise and USS Exeter.
Replaced design one on USS Enterprise.

Design Three
"The Doomsday Machine"

Elliptical housing.
No features.

One could also add the TAS version, which is a rather accurate reproduction of Design One. Would this mean that Design Two was later retrofitted into the earlier version?

The low-res Constellation impulse deck could be an indication that there are shutters on the impulse nozzles, ones that are very seldom closed. Perhaps a (nearly) complete power-down of a ship results in the closing down of the shutters?