FSA Threaded Headset with only one set of bearings?
 

I can't find any other example of this, but it seems to me that my headset only has bearings for the upper race. I can't find any other example of this online. The headset is a FSA Duron. I'm curious to know why I can't find another example of this, or what this history of this type of headset is.

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I suspect you're missing something, either an actual part, or more likely not recognizing one that's there.

Looking at the lower photo, it appears that there's a movable race, which is probably part of a pressed in, or integrated headset bearing. Stick a finger in and see if it turns, and feels like there may be balls behind it. If so, that's the bearing.

In any case, with these and similar designs, the conical parts on the crowns are only centering cones, which engage and licate the integrated baring race.

Right, apparently the lower bearings are needle bearings. FSA Duron Threaded Road Racing Bicycle Headset Part Silver 26 4mm 100g Italian | eBay

Any advice on whether they are worth replacing now that I have it disassembled?

Sorry, that's a poor question. Given that I've been riding year round rain or shine, and they haven't been looked at in a couple of years.

Given that I don't see any rust, I'd just clean and grease them and ride another year.

looks to me like the lower needle bearing race is missing, but maybe I am not seeing something?

Your first (leftmost) photo shows the cartridge bearing still in the race. "TH Industries" is the bearing maker and, incidentally, FSA parent company.