I did some contracting for a retail bank a good while back and the COBOL source for the term deposit interest calculation routine had been lost a few decades earlier but was still in use. I suggested a rewrite but there was no enthusiasm/support for it - especially as my prototype could not exactly reproduce the rounding. Nobody wanted to have to deal with customer communications to explain why one cent of interest was being paid a month earlier or later than before.
Ahaha I'm also working with a bank and "customer communications" is a really nice euphemism for "barrage of class action suits". I'm sitting here in the office laughing out loud.
In the 80s (and probably before), every system I worked on had at least one critical COBOL program missing source code.
I am wondering if you noticed the same with this project.