This, one of the latest model dual master cylinders, was introduced in 1985. One of the notable features is the failure switch on the reservoir cap. This master is now also the only recommended replacement for all models from 1980 thru 1992, replacing the GMC173/198 and the GMC192/195. It should also be noted that this master has metric unions (top is 10mm and the bottom is 12mm) and the pipes in this front rear split master are reversed from earlier models; i.e., the bottom pipe feeds the rear brakes and the upper pipe feeds the front brakes.
If you are replacing an earlier master that had UNF pipe ends fitted to it, you'll need parts HU9 and HU2.