I do not believe there is any way the postcard is from the 20's, but I do believe it is very likely from the 1934 time period. I also believe it is likely to be Gehrig.
Below is a brochure promoting St Pete from 1934 and the Yankees along with Gehrig specifically, is used as part of their promotion. It would serve to reason that St Pete would also make postcards to sell around the same time frame.
I don't have one in my collection as there always seems to be something else I want to buy when one comes up for sale. LOL
