The original (often called "Metal Knob") Neve 33609 was a rack mounted adaptation of the Class A/B 2264, 32264 and 33314 (broadcast version) compressor/limiter console modules, which were a later incarnation of the fully Class A 2254. All use what's called a "diode-bridge" topology to achieve gain reduction. There are other quality modern compressors in this vein that deserve mention as well, namely the Buzz Audio DBC-20/DBC-M and BAE 10DC/10DCF.
The original all discrete "Metal Knob" version of the 33609 was discontinued by Neve and replaced with the 33609 C version. The C version had a more robust power supply and the amplifiers used the BA640 IC based cards, as opposed to the earlier discrete BA440 cards. The new 640 had more headroom and a faster recovery time after clipping, which was important for broadcast use. Another difference was based on the transformers, the C used Belclere transformers, as opposed to the Maranair/St.Ives of the original version.
Neve 33609 C
The most recent iteration, the 33609 N, uses more modern construction techniques, similar to the 1073N. The transformers are now of the Maranair branded variety that AMS Neve is having custom made for them to the original specs of the earliest version. The look of the unit harkens back to the original "Metal Knob" a bit more. They have added a fast and slow switch to the compressor attack time, which none of the other versions had. The unit remains discrete and has a sound very much in line with those originals. There is also enhanced "I/O" switching on this newest version.
AMS Neve 33609JD