The Altec Lansing 438C is a Vari-MU compressor with included mic preamp and threshold and release controls, part of the latest modification of this model series.
Compressor found on all Beatles songs as a very similar variant of these Altecs (based on the 436B without mic preamp and threshold/release settings), was omnipresent in the Abbey Road studios.
This compressor gives the right vintage touch that is needed, especially when recording with its very colorful preamps, and what color!!
We have three in the studio that we use mainly on bass, brass, overheads or room.