I agree--it's most likely printed from a 3 1/4 x 2 1/4 neg through a slightly smaller mask. The problem with this, of course, is that it was one of the most popular formats between the wars. All the consumer-market Kodak cameras came in this version--the folding pocket Kodaks, Autographics and box Brownies, etc. I'm sure it was equally popular among the Kodak clone cameras too, which would make it well nigh impossible to pin down. The pram is a better bet!
You're right about the orientation; the folders, autographics and even the earlier 'cycle' cameras all had viewfinders that rotated to allow for portrait or landscape orientation.