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Anthony N.When I was a kid there was a card store in Hollywood, Adco Sports Exchange. I believe it might have been the first store anywhere. Goody had a seemingly endless supply of '39 Dimaggio's and Greenbergs, all of them with the sample card overprint.
I think they only occur in the low series. I've got a few in my set, they seem to come up fairly regularly and I haven't noticed any premium attached to them.