In recent episodes of the podcast "The Real Brady Bros", Barry Williams and Christopher Knight have pointed out various episodes where the Brady Bunch Stand-Ins can be seen. Above, the woman seen here in these photos from "Today, I am a Freshman", was Barry's on set stand-in for all 5 seasons.
Below is the stand-in for Florence Henderson, seen here in the episode “Kitty Karry-All is Missing”
Also, in another podcast episode on “The Fender Benders”, Barry says the bailiff here is Robert Reed’s stand-in while Florence Henderson’s is seen as one of the spectators watching the proceedings. They describe the woman on the left here in "Will the Real Jan Brady, Please Stand Up?" as either Ann's or Fiorence's stand in, they couldn't recall, as this woman was not with the show for that long
(A stand-in literally stands in front of the camera to set up lighting and to frame and focus the camera shots. Except for height, they don't necessary ever have to look like the actors)