This led to the question of why I had them. So, I did some poking around on the Sylvania bulb-fitting website, and discovered the answer.
They are taillight bulbs for literally every other vehicle I have ever owned, every vehicle suzanne has owned, and even all the ones I've regularly driven and didn't own -- which adds up to a collection spanning three decades of model years. Just not for the Bronco.
(Luckily, the Bronco had two perfectly-good taillight bulbs; the non-functional one worked fine after I prodded it a bit to reseat it.)
Crossposted from Dreamwidth (original here), with comments. Comment here or there; comments here will eventually be duplicated to there.