What follows is not authoritative; as I said, it comes from observation, not from first-hand knowledge.
Run-off legends come in different forms - hand-written or typefaced - and in different positions.
The various parts of the legend are separated by hyphens. The 'Z' seems to indicate a stereo recording, an 'X' a mono one.
They tend to consist of the matrix number of the record with a few add-ons at the end.
The matrix number is often accompanied by a 'cut number', which refers to the 'stamper' (the metal plate being used to press that side of the record).
CBS, Decca, EMI, Philips/ Polydor, Pye, and (in the second half of the decade) RCA appear to have had distinctive and easily recognisable styles, so a few notes about them may be useful.
There were of course a lot of other pressing firms, some of which had their own particular styles; I've been able to identify some of them - Orlake, and possibly British Homophone and Saga - but others remain elusive.
Making a record involves several stages, and marks can be added at each - it should be kept in mind that once a stamper has been made by one firm it can be taken to a different firm for the actual pressing to be done.
Identification, therefore, is likely to be less than an exact science, and the following guide should be approached with caution. A comprehensive list of the various markings and their meanings can be found at https://If it's at the top of the record, that's twelve o'clock; if it's at the bottom, that's six.The three and nine o'clock positions are where you'd expect them to be, as are all the rest of the hours.The various numbers and markings which are generally be found in between the spirals of a record's run-off groove can be a help in identifying which company made that pressing.Judging by a quick look at singles in my collection it seems that different pressing plants had different styles of run-off 'legend'.While reading the following descriptions it helps to think of the record as a kind of clock-face.