I've seen several 1999-Current F250/350/450 SuperDuties and I've noticed a major different in the front ends that I can't put a pattern to...
Some have a plastic headlight housing with a 9006 Cartridge headlight bulb.
Others use a Rectangular 6054 Sealed Beam headlight assembly.

It's not year based, because I've seen several of the same model year with the different front ends.
It's also not trim level based, as I've seen XLT, XL, and Base models all next to each other.
(All fleet vehicles, so I don't think any customization or upgrading was done.)

Was this an options package?

 

I think the entry level SDs had the sealed beams. I have seen them on fleeters too.

 

^

That's the thing. I HATE plastic headlight housings... I usually buy vehicles that are 10+ years old. The plastic is never clear, and usually yellow.

I PREFER the sealed beams...

Plus, I've found retrofits for the sealed beams that incorporate a proper projector so that you get the right cutoff when using HID bulbs. (Another thing I hate is idiots running around with a Xenon bulb or a fake Xenon painted bulb in a halogen housing.)

 

AMEN brother!