Counterfeit illegal club dice have been a problem for a while. They're taking vintage unmarked dice and stamping them with vintage dies. This according to an individual who claimed to have been involved. He pointed out some in my own collection a few years ago. The white "Chicago style" seem to be the most abused.

If they're using modern materials, that could be a give away. The earlier nitocellulose dice will not weigh the same as later cellulose acetate. Of course a lot of collectible dice aren't really that old. Some sign of chemical changes in the dice should be noticeable in 50 year old dice. I tend to be wary of any dealer that seems to have vast supplies of pristine early examples. Also, vintage gold hotstamps stamps age to brown, so they usually go with a black stamp.