Have you noticed any pingy sounds coming out of it? Something like here:
Also, have you tried lubing/silencing the sliders?
Ah, cool comparison to bring up. So for those that dont know, these old Nissho keyboards have quite a bit of metal in them. They are built a lot like Model F's, with thick plastic tops and metal bottoms. This creates a much higher pitched sound than the modern Hi Pro, for example. Think of the difference being like that between a Model M and Model F.
In the case of this case (pun?), the sound is coming front the plastic caps hitting the top of the slider housing. No silencing can prevent this sound, as all the standard Topre silencing is done on the underside, to prevent noise on the upstoke. In my live stream i did use hypersphere rings on the entire top row, and there is absolutely no difference in sound.