I doubt it would be foreign marbles, the cost of imported marbles would be much higher than marbles purchased closer to home, so it would only make sense to import high end hand mades, not common clearies and such. The four makers I know that were operating near PA, cac, akro, pelt and alox. Of that, akro had been making marbles the longest of those 4, so I think they are the most logical. Could be transparent corks you are seeing, which I'm sure could look like a cats eye in a black and white photo from the 20's. Another possibility is a mix of several companies, purchased through pressman or gropper or another seller