Nailhead hit a good part of it there - Cast Iron can be cracked, and then it wouldn't 'ring.'
Keep in mind that a whole one or two generations gave up on cast iron for cookware, and I'm sorta afraid quite a few were used afterward to melt tin and lead in for various purposes.
Pawn shops are nearly experts and valuing what they have on hand because they can easily make more money on ebay if something is collectable. "Looks and feel" old doesn't mean they are old. I read that for a while some CI was been imported from Asia.
But if your wife isn't a stickler for being able to prove it is old, probably any of it will work anyway. I have a totally unmarked skillet I use a lot, and with seasoning it's about as good as any other.
* This is just an opinion, but for modern day non-gas stove tops I tend to believe that while the CI skillets with heat rings are collectable, the flat-bottom ones work better with electric ranges. I have examples of both, and it's apparent that the electric heating coil's inner coils aren't touching the center area of the skillet that has a heat ring (because the heat ring is what is sitting on the outside area of the coil).
* If I had a gas stove, though, it hits me that the gas ring could help trap heat up under the skillet better.
* Disclaimer: I've never read any of that on any CI site, nor in any commentary, but it's just what I've been thinking as of late when I've seen the red-hot inner coils not effectively transferring heat to my heat-ring skillet.