I only know details at the level of war story, secondhand. I may have overstated a bit. For sure there was a build process relinking in old object code for many years which no one knew how to reproduce. Possibly there was still associated source code, but the object bits had been declared "golden" and no one knew exactly what source version or build process had produced those "golden" bits.
I've had something similar before at a prior employer caused by bad linking of some C# to some C++. Someone thought it was a good idea to P/Invoke directly the mangled names. Needless to say that changed once I was finally able to get my hands on the source.
Likely the source was used to build an object library which then got linked in to form the executable. If the library Just Worked, there would be no reason to rebuild it.
How would you go an entire decade without noticing this?
Wouldn't you have to use that missing source code for something within ten years?