It’s worth taking a look at an eBay listing for a used Revere Ware piece to see what we can learn before we buy. Take this skillet for instance.
When I see “unmarked” I immediately think two things – it is either a knock off, or it has been used enough that the copper layer has worn down past the stamp.
This piece, with the distinctive two screw handle and knob, is clearly not a knockoff.
So what about the bottom:
Definitely no stamp, so this piece has been used plenty, and some of the copper has worn off. It is probably still thicker than the post-1968 cookware, but still something to be aware of of you are prizing a piece like this for its thicker copper layer.
However, notice the wear pattern on the bottom:
The fact that the rim around the edge is showing some wear whereas just inside of it is not, is indicative of a warping upward of the center of the pan. Let’s look at the inside:
The way the light is reflecting, almost guarantees this skillet has a pretty significant warping upward in the center. Probably not a great skillet if you value a flat bottom. Here is an example of a skillet on eBay that looks to be much flatter, with little, if any, warping.
If you are buying a used piece on eBay, it may be worth asking the seller if the bottom is warped at all; if they say no, and it come warped, that gives you some recourse t0 return it.