Personal Computer Joystick and Pass Through Component - Dismantled for reference and curiosity
Released in 1984 by RAM Electronics Ltd. (also known as Fleet Electronics Ltd.) the 'RAM Turbo' was designed to outdo the official Sinclair ZX 'Interface 2' with which it shares and similar casing. Regardless of the name, the RAM Turbo doesn't add any RAM to the ZX Spectrum computer.
The Sinclair ZX Spectrum doesn't natively have any joystick ports and instead relies on keyboard input. In 1983 to address this Sinclair released the Interface 2 that adds two joystick ports, a pass through interface and a ROM cartridge port.
Sounds great but in reality it's pretty limited. The pass through interface is modified to only allow use with the Sinclair printer and the joystick ports don't support the popular (at the time) Kempston or Protek joystick protocols.
The ROM cartridge port does work as advertised but as the cartridges only support 16kB of data and most Sinclair ZX Spectrum systems were shipped or upgraded to 48kB of memory they weren't very desirable.
Released only a year later, the RAM Turbo addressed the two main issues with the Interface 2. Its joystick ports support Kempston (right port), Protek (left port) and Sinclair joystick code standards. This means pretty much any 9-pin joystick wired in the Atari 2600 switch to wire set up will work. If you hold down the joystick button during power on it enters Proteck 'cursor mode'. Its pass through interface is unrestricted so users can plug pretty much anything into it that the Sinclair ZX Spectrum can be made to support.
Later models released after December 1985 also include a reset button on the side.
Considering how junky many third party hardware add-ons are, this is most certainly an exception.