I dont think it does fit... If you check out CoffeeParts Naked portafilters:
This is a generic (non OEM) PF modified to be bottomless..... and La Cimbali has a unique part number (several others are shown as sharing the LM portafilter......)
Id give Pedro a ring at CoffeeParts and he will be able to confirm - but Id imagine they have a different thickness lug, or the lug is a different distance down from the lip etc... You could possibly butcher the LM to make it fit---- but why wreck a beautifully made PF?
And personally, I find the La Cimbali genuine PFs are fine and (other than the kudos from owning an LM PF) it wont improve your coffee....
People have had an improvement with a bottomless PF however - probably due to less interference with the crema flowing into the cup (and you can see instantly if you have the dose and tamp correct)