I'm confident it's authentic and from the 1930s. Early 20th century stereviews were curved like that to facilitate the 3D view through the viewer, and I've seen that Ruth steroview before. Also, a prolific seller of antique stereviews would be the perfect person from which to buy a baseball stereoview.
It is about the cheapest 1930s Ruth collectible one can buy, but I'm confident it's genuine. There are much better looking Ruth stereoviews, but they are much more expensive, up to perhaps around $300. Also stereoviews are borderline baseball cards (I said borderline, let's not start another 'What is a baseball card?"debate). They are photos on cardboard, were sold commercially to the general public and people collected them. Personally, I wouldn't call it a baseball card, but it fits within the hobby general 'genre' of baseball card collecting that includes 1911 Sweet Caporal Pins, Tobacco silks, Piedmont Stamps and Topps test-issues. It's as much a card as many of the non-card items included in the PSA and SGC set registries.
I'd say it's better looking than a couple of his strip cards
