Thank you all for the suggestions. I have an entertaining postscript.
Should have added previously that my passenger rear seatbelt appeared to be unable to pull forward after removal, locking with every attempt. I struggled with it for a good 15 minutes without progress. I decided I must have damaged it in removing it. This lead to my attempt to disassemble, thereby causing the belt's subsequent auto-destruction.
Well, I was able to source a used replacement belt from a dismantler, which arrived today. But, same issue! The replacement belt would not pull forward!
I'm cursing the dismantler for sending me a faulty belt, and I start trying to find the part number on the retractor. I see embossed on the plastic cover "Y = 59 X = 0."
Of course. The inertia mechanism must be in the installed position to operate.
I fitted the retractor into the mount. Works absolutely perfectly.
It was an $83, and quite humbling, lesson.