1/2 screen on 1953 Zenith TV
I've seen this with solid state drive and something similar may apply. If it uses push pull or double sided vertical deflection then the driver on one "side" may be dead. In your case presumably a pair of tubes or maybe a double triode or ... .

So ... I'd look at where the deflection drive leads come from and see if there are two identical tubes driving it - presumably via a transformer as they only make vaccuum tubes in NPN  / N-Channel  :-). It could be any of a good range of things causing the problem but IF there are two separate tubes try swapping them. Check plate and grid voltages. A good guess is that a grid blocking capacitor may be very leaky. 

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