Also - especially in the case of Studio One - they would change the artist name to that of a band or singer who'd just had a hit, so a Heptones might be credited to Ken Boothe and a Ken Boothe to Alton Ellis.
And as Dave said, some producers would come over, go see Pama, license them a tune then go across town and license the same song to Trojan, so you get things coming out on different labels with artists names changed (Pat kelly credited as Wonder Boy, Lester Sterling as Mr Versatile etc)