Jay, you beat me to it.... yup no "rookie" cards way back then.
Here's something to ponder... Topps first year for BB cards was 1951. Could you imagine if Mantle was on a blue back Topps in 1951, let's also say it was an SP to boot. I wonder which card would be more desirable, the Topps or the Bowman?
Thank goodness Joplin didn't put out a minor league card of the Mick in 1950.... where would we be then?