I tried some Google searches to find catalogs from Soborg Mobelfabrik from the 1970s, but no luck.
Earlier this year Søborg Møbler was acquired by Carl Hansen & Søn:
I wondered about the plywood seat too.
Thank you, Leif and Zephyr. I like the chairs and want to learn more about them.
Here's the same chair with a footstool, also attributed to Mogensen.