In a matrix code such as 857978 2.1:*:* EMI SWINDON, the black portion of the matrix code is the master ID and the red portion of the matrix code is the stamper ID. In most cases the master ID is burned onto the glass master at the same time as the audio data, while the stamper ID is applied to a mother or stamper made from that master. They can be distinguished by the different ways they reflect/refract light: the master ID is created with the same pits as the audio data and will refract light into a "rainbow" just as the audio data does, whereas the stamper ID is applied mechanically and does not.
All pressings with the same master ID are digitally identical regardless of the stamper IDs, so rather than list every known stamper ID, we use a placeholder.
(On many Japanese releases, the master ID and stamper ID are both added mechanically after the glass master has been created, so it is not possible to distinguish between the two.)