At Uni as a summer job I worked processing Corporate Actions for a large custodial bank. We used exactly the same kind of system where every action was 4 characters. I can still remember some of them despite it being 10 years since I did that job. Even more importantly, the screens were trivially scriptable so lots of the grunt work could be handled by writing export scripts, pulling a bunch of data into excel, processing it and occasionally posting the results back the same way.

Absolutely no way a modern system could be half as efficient, short of completely automating the whole job (which involved a lot of communication with other parties and basically freeform restrictions).