I'm going to go against the grain on this one and say that it might just be an obscure factory option, or otherwise done at the dealership before sale. The Monaco 500 in my sig has been in my family since it rolled off the lot way back in 68', and it has side moulding nearly identical to what yours has. I've seen pictures of it at the dealership when my great aunt bought it and it already had that moulding/ trim running down the sides, so no aftermarket treatment so far as I know. I'd say it is very likely original, even if may have been a dealer add-on and not a true factory option. Good luck with the search, and congrats on that great looking C-Body!