I have a '50's vintage 151 which I am still fettling. The bed for the cutter is basically rough cast and the paint was the leveling agent. After flattening the blade (major milling machine marks all the way to the edge, which had been sharpened convex and I squared up) I still have trouble with some chattering on flat hardwood. Still working on the blade bed which is very difficult to get flat.
From the reading I have done, (and I am no expert): I don't think Stanley makes a convex spokeshave. Also, if you can find one with an adjustable mouth, it will be a better tool.
I have had good luck with a flat blade making hammer, maul and ax handles (I start with a draw knife). With a thin cut adjustment on the spokeshave for the finishing work (a couple thousandths) , you can get a smooth rounded surface. If you want it perfect, a cabinet scraper will finish it nicely without any sanding.
My novice $.02 worth. I'm sure their are others more experienced and knowledgeable that will steer you to what you need:lol: