Originally Posted by
Peter H
No guarantee of accuracy, but see aviation.stackexchange.com/questions/65258/when-was-the-speed-trim-system-implemented-on-the-boeing-737
>Was the Speed Trim System implemented on the original Boeing 737 or just the Boeing 737 NG series?
Boeing's patent (US4676460A) for the STS was filed on 1984-11-28, the same day of the 737 Classic (3/4/500 series) entering service.
So since the addition of the bigger engines on the Classic, it's been there.
According to that answer on Stack Exchange, speed trim on the 737-300s without EFIS is different, it doesn't operate when the flaps are up.