In fact, I've had production code making use of it for over a year now.
The number 558 is also the production code of the episode.
The above diagram shows the relationships of unit tests to production code.
Someone who works under some supervision and occasionally writes production code.
For practical reasons, these specials continued to use series 4 production codes.
In fact, they were the films that led to the Production Code to begin with.
It is important that such testing hacks do not remain in the production code.
The film appeared before the enforcement of the Production Code.
The production codes in the episode table reveal the originally intended order.
Needless to say, the Production Code died a long time ago.