It is a duct detector. I’ve posted the photo w/ the model # shown, in case anyone has any info about it. The round, black bit in the lower right area has 4 holes in it, 3 of which have wires attached inside the box. I found that shorting the top left & bottom right wires (by sticking a piece of 18AWG wire in those 2 holes) the unit goes into alarm. Still, there must be some other way to test these things.
Knowing nut luck lately, I’ll locate a “test key” of some kind the day after they take my advice & replace these w/ the Simplex duct detectors they’ve got in storage.