Early 1950's marbles (I think) with a bubble in the center of them not sure if this is common or not?

Not sure but I think these marbles are from the 1950's? I don't know much at all about marbles. But I do have a few for sure and as I was going thru them I found 4 of them that have this unusual bubble inside them. I wish someone could tell me if they're from the 1950's or not and if it's a common thing to acquire marbles with a bubble in the center of them like these pictured here? All input is much appreciated and thank you in advance.