You got it, Paul!

These tubes are in fact spent 75mm cases. This information comes from Norman Avery's book again (page 114). They used the cases as covers for the 0.5 guns against the fine sand on the Gilbert islands. Below are 2 photos with those covers:
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And the same a/c as above without the covers:
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I believe what we see in the very first photo you posted are short stubs attached to the nose and the "covers" have been attached to them after the guns have been installed. Of interest are the gun barrels in a lighter tone. I don't know what is the reason for it but on the next photo one can see that the whole gun looks like painted over (it's obviously not just dust):
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Unfortunately I don't know the name of the a/c you've posted (Lil something) and couldn't find it in the rest of the photos from Tarawa I have.
I don't remember any details about the modified strafers with a 20-mm gun. I'll check some sources, but I don't think I've seen a good description of this modification.
Cheers!