Marilyn Qualmann went missing on Sunday 21 September 1975 from her home on Moorook Road, Moorook (via Kingston-on-Murray).
The last confirmed sighting of the 14-year old was at about 9am on that day by her sister.
There were suggestions that Marilyn had simply run away from home, but investigators believe something more sinister happened to the young girl.
She was reported missing the next day and has not been seen since. She is suspected to have been murdered.
A reward of up to $1,000,000 remains on offer to anyone who either provides information leading to the recovery of their remains or information leading to a conviction in the case.
Anyone with information is asked to contact Crime Stoppers on 1800 333 000 or at www.crimestopperssa.com.au