It's all very well having these highly complex machines capable of recording and replaying, but there is another essential component of the whole system and that is the tape.
As well as the spools and cassettes there is a vital need to know what's on them , where they are stored and, after use, to recycle them for re-use.
In 1980 an important document which specified how the Programme Number should be used was issued to everyone. I found my copy and you can view the pdf below (or download it if you want to!)