Ted,
The barrel flats indicate that they are slightly overbored for the respective gauge. I've had the bores measured as well as the chokes. The choke measurments indicate a very long choke taper. I suspect that is why they pattern as such.
RE: Spreaders. Years ago I was on a hunt with a couple of guys, one who was using spreaders. The dog took a pellet in the lung with deadly results. That hunt still haunts me.
Karl