Hi,
I don't know the model you mention, but important to know is how the 'switching' per key is done :
a. multiple contacts moved by a kind of strip
b. only 1 contact with electronic components to multiply them for all voices.
c. diode matrix switches

When you say 'old organ' I expect 5 or more gold- or silver plated springs or wires all moved simultaniously up/down when the key is pressed or released.
If this IS the case I expect some contacts to be defective.

In old organs the tone-generator produces only the highest tones from C to B. To get all others a simple devider-circuit is all you need. (e.g. C3 devided by 2 gives you a C2 etc.)

So your organ probably has some mechanical problems. (switches)

Roel