a pair of Laurel brass wishbone & wood torchiere floor lamps  

It is most likely not brass, ...
It is most likely not brass, simply a lacquer that was used to give a brass colored finish. I've seen it on Laurel and Thurston lamps to name a few and was commonly used in the 50s-70s. It comes off quite easily if you try to clean it.
Many people will call it brass plated without actually investigating, just assume.
I can't see all the pieces you are referencing to judge, just the 1st dibs piece. But in my experience what you are seeing is lacquer - not plating.

What she said.
You can strip the lacquer off with lacquer thinner or acetone and paper towels or #0000 steel wool if the metal has a brushed texture.

thanks!--I think you both must be right--
the one I bought has a surface that looks very much like dull chrome (I'd photograph it, but reflections make it impossible to capture)--so someone presumably stripped it completely after the lacquer began flaking off.
anybody know if that lacquer product still exists?

I always thought it was
just regular lacquer that had yellowed over time, not something tinted to make the metal look like brass.
Though I have seen plenty of tinted lacquer on wood furniture. so maybe it was tinted lacquer, I dunno.

no, this is unmistakenly tinted--
the finish looks extremely even.
I wonder what the metal underneath would likely be? Non-magnetic, so....zinc? aluminum?
the keywords [laurel wishbone lamp] bring up the examples I'm looking at on the Bay....

ok, I've now acquired a "brass" table lamp
....of this design, with a brilliant finish, just like the ones on ebay etc.
I hit it with 0000 steel wool along the side of the base (a strip about an inch long and a third of an inch high).
The steel wool doesn't strip it. It polishes it, mirror bright.
EDIT: no, it strips it!
Underside of the base is a grey/silver metal.
Neither side is magnetic.
So what's the metal? Any ideas?

here's the base of one currently on the bay
....this is how all the ones I've seen look:

and here's the fork of mine
midway through the steel wool process...

Pot metal
Look up the term pot metal. I believe this is what you are dealing with. Wikipedia has a decent overview. Very common in lamps.