Introduced with the launch of the 00 scale game in the 60s, previously runs had been measured by the number of flicks taken to return the ball to the wicket stop, divided by two.
One score determinator was included in every box set and they were also sold in the accessories range in a package with the rules book as item TC-K.
Three variants can be found. The white with imprinted writing version (left below) is solid plastic. The white with black writing is far thinner and more flexible. Although this looks more modern, it appears to crop up in late 60s Display editions most commonly.
As with many cricket accessories, where there’s a white version there’s also a green one. Again imprinted solid plastic. These are less common, but by no means rare.