my 83 had the single reservoir master cylinder also and the original reservoir had a plug on it as you can see in this pic, there is the cap, then the other black thing on top was were the plug went for the fluid level sensor. when i got a new reservoir it was not on there anymore. i think someone replaced yours before that why its not there anymore. and the other hose is for the rear window washer at the top of your rear window. there are 2 plugs below the hose that you dont have circled, i believe cause you know they are for the washer tank