Charles said this was his second professional acting job, and had fond memories of all The Brady Kids, Susan Olsen mentioned that he also worked with them on "The Brady Bunch Visits ABC". It's tough to find any video of this, but Smith mentioned it in this interview with Media Mike.
MS: Your first television role was as the young man who sold Greg Brady a junk car on “The Brady Bunch.” Have you ever thought about buying something and then thought “caveat emptor?”
CMS: Caveat emptor? Ha, no not really. That was technically my second TV role. My first was a special also involving the Brady Bunch. It was made by ABC to advertise their new season of Saturday morning cartoons. I got those roles right after my first professional acting job, “The Culpepper Cattle Company”, a western for 20th Century Fox.
