It honestly looks like a saw guide to me, or some sort of similar guide for sawing straight lines. Does that tab on the top match the width of the tracks in a table saw? And in the first photo, I think I detect a faint line near the bottom of the plate, perhaps indicating it's been sliding along a slot.
The fittings on the ends of the threaded bit do not seem appropriate for a brake pad spreader, and that knurled knob looks more like it's used for adjustment; it does not seem big enough to apply the amount of force needed for brake pads. There's also no reason a brake pad spreader would need that tab along the top of the plate.
And of course I could be all wrong.