All 4-string basses in my collection have a disc shaped string retainer, as seen on this L-1000 from 1980, the oldest bass in this collection, as well as a gold plated version on one of my ASAT Bass Commemorative Editions. But most 5-string models seems to be using the same U-shaped bracket for a string retainer as used on early guitars. Except for this 1994 L-5500 that is, which has a 4+1 tuning machine arrangement and a disc shaped string retainer.