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I have a '73 and mine doesn't have one either. I plan on installing a shutoff in the water feed line at the intake manifold before I put it on the road in the spring.
You can get an inexpensive discrete brass shut off at any hardware store. Just remember to turn on the flow of water to the heater core periodically to circulate fresh coolant.
Several folks have installed some type of valve to shut off the hot water flow to the heater core. I use a dual valve from a Ford Ranger as shown in my gallery.