Slags were a different method in an earlier period. Slags were discontinued and corks were introduced at the tail end of 1929 or the beginning of 1930.
In the 1930's though, there were lots of marbles which weren't corks. There were patches, sparklers, moonies, flinties and as you say, game marbles. I'd guess they had different set ups for those which didn't have a spinner cup attached at all.