Perhaps it would be useful now to clarify what we know regarding these amphisbaenic rhymes.
We know:
1) VN read Wilson's "Pickerel Pond" poem sometime in 1948. He then responded with a clever poem of his own, which he included in his 21 November 1948 letter to EW. It reads:
To E. W. on reading his amphisbaenic poem
At first my brain was somewhat numbed
by your somnambulistic numbers, Edmund.
Now, having shaken off that stupor,
I find the latter anagrimes with "Proust"
while "T. S. Eliot"
goes well with "toiliets."
O Emir
of the mirror-rime!
I fear your envy,
and humbly sign: V. N.
2) In a 6 Feb 1949 letter, VN offered a new quatrain:
Do you still work upon such sets
as for example "step" and "pets,"
as "Nazitrap and "partizan,"
"Red Wop" and "powder," "nab" and "ban"?
3) In a 23-25 May 1949 letter to EW, VN says, "I never got any thanks from you for the wonderful emir [rime] I gave you: partizan--Nazitrap." Therefore, we know that this rhyme was VN's invention.
4) Looking back at EW's poem, we can see that EW used the step/pets rime, but not Nazitrap/partizan, Red Wop/powder, or nab/ban. We also discover that EW used stops/spots and spider/redips, both of which (the former sans sibilants) VN borrowed and gave to John Shade's "Pale Fire."
5) We should also notice EW's reference to Nova Zembla. Could this reference in an amphisbaenic poem have inspired VN to think of Zembla as a "mirror-land"?
6) We still don't know the origin of Red Wop/powder or nab/ban, but since they come after the Nazitrap/partizan rime, which we know VN invented, we may infer that these too are VN's contributions.
After all this, we still don't know for sure what Red Wop meant to VN. We have, however, learned that Wilson's poem contributed something to "Pale Fire" (spider/redips, pot/top). Indeed, there are a number of amphisbaenic figures in the poem, even beyond those employed by Hazel. My favorite: how "Retake, retake" (487) turns round to become "skaters" in line 489. Almost as if JS is trying to rewind the scene.
Matt