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It's hard to read the schematic well enough to tell, but two thoughts come to mind: 1.) the zener may be meant to keep the tuner signal under 8 volts in the event of AGC failure to protect the tuner. 2.) they are using the fixed voltage drop off the zener to increase the DC voltage level going to the tuner relative to the IF AGC bus.
Once you start looking at SS gear schematics you start to find all sorts of clever ways to take advantage of the new properties of SS devices.
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