I did a little searching around on the web but couldn't find any reliable references suggesting that Wallis was awarded a cypher in the usual fashion. Edward, as a HRH and a Duke of the Realm, obviously was entitled to a cypher - the "E" with the coronet above it. Wallis adopted / used a similar one, with the E turned on its side to form a W, again with the coronet above.
Then again, this is a couple who bent rules to suit themselves. Example, within their household the duchess was addressed as HRH, even though she was not entitled to be. So her use of a cypher - with or without entitlement - is no surprise.