My dad put that same fixture out in his back yard, and recently converted it to two 26 Watt daylight CFL

This looks like a fixture with a motion detector.
CFLs shouldn't be used with motion detectors.
In Israel, we have halogen floodlights with motion detectors such as this one (Most of them have a photocell as well). All of them uses R7s double ended halogen lamps.

I failed to mention that my dads fixture is a dusk to dawn.  Not a motion sensing light.

So what is the sensor at the bottom of the fixture in this video?
It can only be a motion detector, since it have the shape of a motion detector.
Dusk to dawn sensor, should be located on the top of the fixture.
Most halogen floodlights with motion sensors in Israel, have a dusk to dawn sensor as well.

All variants of fixture may have the same box, only part of it actually populated - so either PIR only, light only, or both.

Even the dusk-to-+dawn only may use the same shape as the PIR, simply because with that you suffice with the common box (so using common plastic mold, what is very expensive, yet still very special tool in the production). And that save a lot of money for the dusk-to-dawn only version, as it is not as common, so sold only in rather small quantities (when the bigger box and the Fresnel lens, even when not needed for the functionality, are then cheaper then special mold for the dusk-to-dawn only version). More over there are way more things common for both versions, so the unifications make the things cheaper and logistic wise simpler to handle in the production.

Well said Medved