Modified 3 band compressor/expander with vairiable control. Ground, power supply, and input gain modifications. Nice controls. Excellent unit.
The 503 is simply the rack mount version of the original 3bx expander design with 2 variations of circuits. 1 version 3bx used all discreet circuit parts and actually can sound a little tube like with some harmonic "thickness" due to the VCA design. This is a very rare version. The later second version used a custom made dbx IC used in most vintage products. Both 3BX are known to sound robust in comparison to modern 3BX XDS which can sound a little forward and slightly bright.