[Accelo]

It looks like the hump cover on you truck may have been replaced. I am assuming there is no other provisions for any type of parking brake. I am confident a parking brake was required. This setup was likely used to avoid the complexities of adding a parking brake to the rear wheels. I have seen this on school buses and all sorts of heave equipment. I agree it may be rare on a C20. It appears to be the correct transmission as it matches the SID. These transmission typically outlast the truck they are in. Another reason I believe it's likely an original. it is unlikely GM would have used more than one method of operation for the same brake.