Lenslok (lenslock)

Lenslok is a copy protection mechanism found in some computer games and other software on the 8-bit Atari computers, Commodore 64, Sinclair ZX Spectrum, Sinclair QL, MSX and Amstrad CPC. The most famous game to use it was Elite for the ZX Spectrum.

The Lenslok device was essentially a row of prisms arranged vertically in a plastic holder. Before the game started, a two-letter code was displayed on the screen, but it was corrupted by being split into vertical bands which were then rearranged on screen. By viewing these bands through the Lenslok they were restored to their correct order and the code could be read and entered allowing access to the game. The device was small enough when folded flat to fit next to an audio cassette in a standard case. – Wikipedia

Lenslok was used on Elite and OCP Art Studio for the Commodore 64
There is a Lenslok emulator online here:
http://simonowen.com/spectrum/lenskey/

This is what the device looked like and how it’s used