There rarely is an original specification, and even if one exists, what was actually written has a high probably of not actually matching the specification exactly because of undocumented feature requests or additions.

You did read the bit that said the current system is the accumulation of almost 60 years of changing tax legislation, right?

Yeah, ~30 year projects were the longest i've ever worked on. Sometimes it's easier to read the docs about what the old programmers were trying to do - those telephone sized documents explaining requirements. if you're lucky, they might have started using source control in the 90's and you can kinda match up specs to commits.

> you could also just dust off the original specification.

You must be new here. What specification? Oh dear.