probably a silly question but i’ve always wondered why it is ‘2’ and not ‘1’???
It got cut out by the highpass filter. 
Filter 1 (though it would just be called “Filter”) was included in early versions of Renoise. If you load an old song which uses it into a new version of Renoise, I believe the old filter will still actually appear, but only under those circumstances. You can no longer load the filter directly for example, it’s just there for compatibility reasons now.
You can see it in this old screenshot for example:
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it has been removed from available DSP effects because the new filter is more performant and sounds better. The old one was bugged, in one word 
But does it still load with really old songs? I seem to remember reading that in a thread here some time ago.
yes, your previous statements are true, so I just added some more facts: Filter1 is still being loaded in old songs because it sounds different than Fitler2
ahright, thanks for the replies.