http://alienbees.com/plm.html
They added this section fairly recently, i think I know what i'm doing now 
It can be used with both, I was wrong. Though I don't think it fitted very well when I tried it on the silver as opposed to the white. Often I have trouble with it falling off, in fact. I don't often use the black spill kill cover, though I suppose I really should.
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White Front Diffusion Fabric: Converts the PLM into a soft, high-output softbox when used with the black/silver umbrella. When used with the white umbrella and Black Outer Cover, it produces a very soft, wide-angle softbox. Available in 86”, 64”, and 42” sizes.
Not sure if these examples are good enough or not but...here is the white PLM with black spill kill cover on the back:
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And on the
right here is a white PLM with NO black spill kill cover on the back:
Same backdrop ...though I suppose with the different shoot it's not really a comprehensive enough test. I can see it a little bit but only because they're mine. Very soft on the left, and a bit more contrasty/directional on the right..Sorry if they're not good enough :\ Both are just the single light