I've had something similar...
with an electrophysiology amplifier that was giving someone some gyp. They'd come to me on a computer issue and seen all the electronics gear on my workbench. "Do you know anything about electronics then? Because we're getting some chronic noise on a pre-amp."
I said I'd take a look and lo, they'd mismatched the impedance on the headstage to the pre-amp and subsequently had the gain right up. I pointed it out and they said they couldn't change the input impedance. "Yes you can", I replied. "No, you can't", they said. "Look... no switch."
I pulled the module and showed them the jumper on the board.
"Yes, you can." And pointed out my initials which were worked into the tracks on part of the PCB.
I had done some improvement work to the company's original pre-amp about 20 years ago, and gave a copy of the design to the company rep next time I saw them. No charge. It was low volume, quite specialised equipment, and what I had done was only a fairly obvious improvement to the common-mode noise rejection circuit which let the user fine-tune the pre-amp to match a particular headstage and correct for notoriously variable experimental set ups, but they incorporated it into the Rev B boards for the next 5-10 years or so. Out of vanity I had arranged the components in the shape of my initials and that found its way into the production model.