1953 Chevrolet 3100 series.
... but the last time I was under the truck, one of the lines runs into a big giant nut on the back of the master cylinder...
That is the end plug. It is threaded into the main-cylinder casting. In this era there was only a single hydraulic circuit so one typically finds a tee-fitting threaded directly into the end cap - with two compression fit lines running into the tee. One leg feeds the drivers front wheel - the other leg goes to the passenger side where another tee fitting splits to feed the passengers side wheel and the long line to the rear wheels.
Manual pages and pics can be found at tocmp:
The Old Car Manual Project - 49-54 Chevy Section 5
