LPs of The 1960s
For LPs the label chose the prefix ILP (meaning: Island Long Player) with a number of three figures beginning with 900, later supplemented by ILPS (meaning: Island Long Player Stereo) to distinguish between Mono and Stereo records. Some records were issued in both forms. For those records in stereo, the catalogue number was changed by adding a fourth figure (e.g.: ILP 970 was the mono equivalent of ILPS 9070).
Read more about this topic: Island Records Discography