David Butleradded December 12, 2019

The Exactra 20 is a rare calculator introduced by Texas Instruments in April 1974. Like the rest of the Exactra line, the Texas Instruments name is hidden away in small print on the back. These were meant to be low-budget calculators. They may have been exclusive to specific university bookstores, but I am unsure.
This calculator includes a switch between '$' and 'F'. When choosing '$', you get 0.00 on the display. When choosing 'F', you just get '0'. This calculator was rereleased in Europe as the TI-2000, but they removed that switch. Instead, it is hardwired to one position. I believe it uses the 'F' setting.
This calculator has a display with seven positions. The furthest item to the left is used for either a negative sign or an error, so only six positions are used for numbers. However, this seven position bubble display is very special. Only three calculators related to the line: Exactra 19, Exactra 20, and TI-2000 have this display. There may have been a TI-1900, as a European release of the Exactra 19, but it is so rare that I have never seen a photo.
The reason that the seven position bubble magnifier on the display is so special is because it was used as part of an iconic movie prop. It was the activation switch on Luke Skywalker's light sabre from Star Wars IV: A New Hope. I included a movie promotional photo showing the prop.
I suspect that part of the reason that the Exactra 19, Exactra 20, and TI-2000 are so rare is because people keep pulling out the bubble lenses for making prop replicas. Of course, once someone owns one of the calculators, they are well within their rights to do what they want with it. However, it seems like such a waste to me. Surely, it can't be hard to duplicate the part rather than ruin a vintage calculator. The magnifier is just a plastic rectangle with seven half-spheres on it.