I like to stress that “Magnetkontencomputer” and especially “Magnetkontenaufsatz” is strange, even to a German native speaker (at least of my age).
(“Konten”, pl of “Konto” is German for account, “Aufsatz” means here an set-top attachment and “Magnet” is, well a magnet. I do remember similar systems being used in Banks, but I am not familar with this term. Rather, I would have assumed that these went by the more general term “Buchungssystem”.)
Magnetkonten are not cards but big letter type (A4) papers with magnetic stripes. I think this is a common 70s or 80s office term (of course mainly known there). I think usually not used at banks but mainly at companies for orders/bills bookkeeping. I still have a school book about it.