Thanks Richard. You had good fortune with those chairs, then.
As to the dowels, that's purely a guess on my part although maybe an educated one. I have a copy of an old book that shows construction methods for modern furniture along with dimensioned "plans" for quite a few pieces. By far, the most common joinery method shown is dowels. I know they had machines back then to drill multiple holes at the same time and holes would be a lot easier and faster to drill than cutting mortises and tenons. I think mortise and tenon joinery would be stronger and if I were building it, that's what I would use. Well, I might use the Festool Domino and loose tenon joinery. π
I found that the slats are held in position on the dowels with screws driven in from below. This would make it quick and easy to assemble in a factory setting.