523 "S" boards - general low output moving coil.
523 "K" boards - certain (ask dealer) low output moving coils.
522 "N" boards - general moving magnet.
523 "E" boards - certain medium output moving coils (EMT, Roksan Shiraz etc..). May also be labelled as NA523 "R" boards.
526 boards - line level input
528 boards - line level with variable attenuation and high frequency filter