[Yaesu] FT-990 filters

[Mike VK4DX]

I just bought a second hand FT990 AC. It came with buillt in 500 Hz 
(10.9MHz) CW filter. This one has been described as a "CW-wide" filter 
in the manuals book. There is also place for optional "CW-narrow" 
filter, the book reffers to 455 kHz 250 Hz filter which, by my 
understanding is activated by pressing "250" button on the front panel 
of the radio.

Since this one is somewhat too narrow for my taste i'm thinking of 
putting another 500Hz,(this being 455 kHz one) in that slot.

The question is how this one will be activated ? Will it be activated 
together with the other 500 Hz filter (10.9 MHz one) by pressing 500 
button or just as the 250 Hz(455 would be activated, by pressing 250 
button ?

Also, are there any 350 Hz (455) filters available as this would be 
ideal bandwidth for me ?

Any help will be appreciated.

73 Mike