As arcade video games became popular around the late 70s, several Pocketeers attempted to ape elements of their play - this is, obviously, a take on Space Invaders. You pull a lever to wind up the timer, then the set of three barriers in the middle moves from side to side while you attempt to shoot through the gaps and hit the alien saucer to score points.
 

ORIGIN: UK

VARIANTS: Space Invasion (US)
Pocketvader (JP)

The US version more closely resembles arcade Space Invaders, with the laser base, bunkers and five rows of aliens all looking more like their coin-op counterparts.