You are probably correct - in my case (87 300E) it was a ball point pen. My tech accelerated the car to a relatively slow speed speed (less than 15 mph) in his parking lot, hit the brakes hard to make the pen slide forward, drove slowly into a parking stall, and took out the carpeting in front of the seat and retrieved the pen from the duct at that area (didn't have to remove the seat). Might work for you, and easier than taking out the seat.
I suspect this happened a lot. The defrost slot on the early w124s is wide enough for a ball point to roll into (The pen had my dad's name on it). Later w124s I have seen had a toothed grill in them.
2003 Firemist Red/grey leather SL 500
2015 Palladium Silver/black mbtex GLK 350
1987 Smoke Silver/burgundy mbtex 300E Sportline (SOLD)
Click to see 87 300E